Transmission (medicine)

In medicine, public health, and biology, transmission is the passing of a pathogen causing communicable disease from an infected host individual or group to a particular individual or group, regardless of whether the other individual was previously infected.[1]

The term strictly refers to the transmission of microorganisms directly from one individual to another by one or more of the following means:

  • droplet contact – coughing or sneezing on another individual
  • direct physical contact – touching an infected individual, including sexual contact
  • indirect physical contact – usually by touching a contaminated surface, including soil (fomite)
  • airborne transmission – if the microorganism can remain in the air for long periods
  • fecal-oral transmission – usually from unwashed hands, contaminated food or water sources due to lack of sanitation and hygiene, an important transmission route in pediatrics, veterinary medicine and developing countries.

Transmission can also be indirect, via another organism, either a vector (e.g. a mosquito or fly) or an intermediate host (e.g. tapeworm in pigs can be transmitted to humans who ingest improperly cooked pork). Indirect transmission could involve zoonoses or, more typically, larger pathogens like macroparasites with more complex life cycles. Transmissions can be autochthonous (i.e. between two individuals in the same place) or may involve travel of the microorganism or the affected hosts.

Definition and related terms

An infectious disease agent can be transmitted in two ways: as horizontal disease agent transmission from one individual to another in the same generation (peers in the same age group)[2] by either direct contact (licking, touching, biting), or indirect contact through air – cough or sneeze (vectors or fomites that allow the transmission of the agent causing the disease without physical contact)[3] or by vertical disease transmission, passing the agent causing the disease from parent to offspring, such as in prenatal or perinatal transmission.[4]

The term infectivity describes the ability of an organism to enter, survive and multiply in the host, while the infectiousness of a disease agent indicates the comparative ease with which the disease agent is transmitted to other hosts.[5] Transmission of pathogen can occur in various ways including physical contact, contaminated food, body fluids, objects, airborne inhalation, or through vector organisms.[6]

Routes

The route of transmission is important to epidemiologists because patterns of contact vary between different populations and different groups of populations depending on socio-economic, cultural and other features. For example, low personal and food hygiene due to the lack of a clean water supply may result in increased transmission of diseases by the fecal-oral route, such as cholera. Differences in incidence of such diseases between different groups can also throw light on the routes of transmission of the disease. For example, if it is noted that polio is more common in cities in underdeveloped countries, without a clean water supply, than in cities with a good plumbing system, we might advance the theory that polio is spread by the fecal-oral route.

Airborne

"Airborne transmission refers to infectious agents that are spread via droplet nuclei (residue from evaporated droplets) containing infective microorganisms. These organisms can survive outside the body and remain suspended in the air for long periods of time. They infect others via the upper and lower respiratory tracts." [7] Diseases that are commonly spread by coughing or sneezing include bacterial meningitis, chickenpox, common cold, influenza, mumps, strep throat, tuberculosis, measles, rubella, whooping cough, SARS and leprosy.

Droplet

"Droplet transmission occurs when respiratory droplets generated via coughing, sneezing or talking contact susceptible mucosal surfaces, such as the eyes, nose or mouth. Transmission may also occur indirectly via contact with contaminated formites with hands and then mucosal surfaces. Respiratory droplets are large and are not able to remain suspended in the air thus they are usually dispersed over short distances."[8]

The pathogen-containing particles, also called Flügge droplets (after Carl Flügge), are 0,1–2 mm in diameter, and are reduced by evaporation to droplet nuclei – small (smaller than 100 μ in diameter), dry particles that can remain airborne for long periods.[9]

Organisms spread by droplet transmission include respiratory viruses (e.g., influenza, parainfluenza virus, adenovirus, respiratory syncytial virus, human metapneumovirus), Bordetella pertussis, pneumococci, diphtheria, and rubella.[10]

Fecal–oral

WPA Outhouse
1940 US WPA poster encouraging modernized privies

In the fecal-oral route, pathogens in fecal particles pass from one person to the mouth of another person. Main causes of fecal–oral disease transmission include lack of adequate sanitation and poor hygiene practices - which can take various forms.

Fecal oral transmission can be via foodstuffs or water that has become contaminated. This can happen when people do not adequately wash their hands after using the toilet and before preparing food or tending to patients.

The fecal-oral route of transmission can be a public health risk for people in developing countries who live in urban slums without access to adequate sanitation. Here, excreta or untreated sewage can pollute drinking water sources (groundwater or surface water). The people who drink the polluted water can become infected. Another problem in some developing countries, such as India, is open defecation which leads to disease transmission via the fecal-oral route.

Even in developed countries there are periodic system failures resulting in a sanitary sewer overflow. This is the typical mode of transmission for the infectious agents of for example: cholera, hepatitis A, polio, Rotavirus, Salmonella, and parasites (e.g. Ascaris lumbricoides).

Sexual

This refers to any disease that can be caught during sexual activity with another person, including vaginal or anal sex or (less commonly) through oral sex (see below). Transmission is either directly between surfaces in contact during intercourse (the usual route for bacterial infections and those infections causing sores) or from secretions (semen or the fluid secreted by the excited female) which carry infectious agents that get into the partner's blood stream through tiny tears in the penis, vagina or rectum (this is a more usual route for viruses). In this second case, anal sex is considerably more hazardous since the penis opens more tears in the rectum than the vagina, as the vagina is more elastic and more accommodating.

Some diseases transmissible by the sexual route include HIV/AIDS, chlamydia, genital warts, gonorrhea, hepatitis B, syphilis, herpes, and trichomoniasis.

Oral sexual

Sexually transmitted diseases such as HIV and hepatitis B are thought to not normally be transmitted through mouth-to-mouth contact, although it is possible to transmit some STDs between the genitals and the mouth, during oral sex. In the case of HIV this possibility has been established. It is also responsible for the increased incidence of herpes simplex virus 1 (which is usually responsible for oral infections) in genital infections and the increased incidence of the type 2 virus (more common genitally) in oral infections.

Oral

Diseases that are transmitted primarily by oral means may be caught through direct oral contact such as kissing, or by indirect contact such as by sharing a drinking glass or a cigarette. Diseases that are known to be transmissible by kissing or by other direct or indirect oral contact include all of the diseases transmissible by droplet contact and (at least) all forms of herpes viruses, namely Cytomegalovirus infections herpes simplex virus (especially HSV-1) and infectious mononucleosis.

Direct contact

Diseases that can be transmitted by direct contact are called contagious (contagious is not the same as infectious; although all contagious diseases are infectious, not all infectious diseases are contagious). These diseases can also be transmitted by sharing a towel (where the towel is rubbed vigorously on both bodies) or items of clothing in close contact with the body (socks, for example) if they are not washed thoroughly between uses. For this reason, contagious diseases often break out in schools, where towels are shared and personal items of clothing accidentally swapped in the changing rooms.

Some diseases that are transmissible by direct contact include athlete's foot, impetigo, syphilis (on rare occasions, if an uninfected person touches a chancre), warts, and conjunctivitis.

Brocky, Karoly - Mother and Child (1846-50)
Brocky, Karoly - Mother and Child (1846-50)

Vertical

This is from mother to child (more rarely father to child), often in utero, during childbirth (also referred to as perinatal infection) or during postnatal physical contact between parents and offspring. In mammals, including humans, it occurs also via breast milk (transmammary transmission). Infectious diseases that can be transmitted in this way include: HIV, hepatitis B and syphilis. Many mutualistic organisms are transmitted vertically.[11]

Iatrogenic

Transmission due to medical procedures, such as touching a wound, an injection or transplantation of infected material. Some diseases that can be transmitted iatrogenically include: Creutzfeldt–Jakob disease by injection of contaminated human growth hormone, MRSA and many more.

Vector-borne

A vector is an organism that does not cause disease itself but that transmits infection by conveying pathogens from one host to another.[12]

Vectors may be mechanical or biological. A mechanical vector picks up an infectious agent on the outside of its body and transmits it in a passive manner. An example of a mechanical vector is a housefly, which lands on cow dung, contaminating its appendages with bacteria from the feces, and then lands on food prior to consumption. The pathogen never enters the body of the fly. In contrast, biological vectors harbor pathogens within their bodies and deliver pathogens to new hosts in an active manner, usually a bite. Biological vectors are often responsible for serious blood-borne diseases, such as malaria, viral encephalitis, Chagas disease, Lyme disease and African sleeping sickness. Biological vectors are usually, though not exclusively, arthropods, such as mosquitoes, ticks, fleas and lice. Vectors are often required in the life cycle of a pathogen. A common strategy used to control vector borne infectious diseases is to interrupt the life cycle of a pathogen by killing the vector.

Tracking

Tracking the transmission of infectious diseases is called disease surveillance. Surveillance of infectious diseases in the public realm traditionally has been the responsibility of public health agencies, either on the (inter)national or a local level. Public health staff rely on health care workers and microbiology laboratories to report cases of reportable diseases to them. The analysis of aggregate data can show the spread of a disease and is at the core of the specialty of epidemiology. To understand the spread of the vast majority of non-notifiable diseases, data either need to be collected in a particular study, or existing data collections can be mined, such as insurance company data or antimicrobial drug sales for example.

For diseases transmitted within an institution, such as a hospital, prison, nursing home, boarding school, orphanage, refugee camp etc., infection control specialists are employed, who will review medical records to analyze transmission as part of a hospital epidemiology program, for example.

Because these traditional methods are slow, time-consuming, and labor-intensive, proxies of transmission have been sought. One proxy in the case of influenza is tracking of influenza-like illness at certain sentinel sites of health care practitioners within a state, for example.[13] Tools have been developed to help track influenza epidemics by finding patterns in certain web search query activity. It was found that the frequency of influenza-related web searches as a whole rises as the number of people sick with influenza rises. Examining space-time relationships of web queries has been shown to approximate the spread of influenza[14] and dengue.[15]

Computer simulations of infectious disease spread have been used.[16] Human aggregation can drive transmission, seasonal variation and outbreaks of infectious diseases, such as the annual start of school, bootcamp, the annual Hajj etc. Most recently, data from cell phones have been shown to be able to capture population movements well enough to predict the transmission of certain infectious diseases, like rubella.[17]

Relationship with virulence and survival

Pathogens must have a way to be transmitted from one host to another to ensure their species' survival. Infectious agents are generally specialized for a particular method of transmission. Taking an example from the respiratory route, from an evolutionary perspective viruses or bacteria that cause their host to develop coughing and sneezing symptoms have a great survival advantage, as they are much more likely to be ejected from one host and carried to another. This is also the reason that many microorganisms cause diarrhea.

The relationship between virulence and transmission is complex, and has important consequences for the long term evolution of a pathogen. Since it takes many generations for a microbe and a new host species to co-evolve, an emerging pathogen may hit its earliest victims especially hard. It is usually in the first wave of a new disease that death rates are highest. If a disease is rapidly fatal, the host may die before the microbe can be passed along to another host. However, this cost may be overwhelmed by the short term benefit of higher infectiousness if transmission is linked to virulence, as it is for instance in the case of cholera (the explosive diarrhea aids the bacterium in finding new hosts) or many respiratory infections (sneezing and coughing create infectious aerosols).

Beneficial microorganisms

The mode of transmission is also an important aspect of the biology of beneficial microbial symbionts, such as coral-associated dinoflagellates or human microbiota. Organisms can form symbioses with microbes transmitted from their parents, from the environment or unrelated individuals, or both.

Vertical transmission

Vertical transmission refers to acquisition of symbionts from parents (usually mothers). Vertical transmission can be intracellular (e.g. transovarial), or extracellular (for example through post-embryonic contact between parents and offspring). Both intracellular and extracellular vertical transmission can be considered a form of non-genetic inheritance or parental effect. It has been argued that most organisms experience some form of vertical transmission of symbionts.[18] Canonical examples of vertically transmitted symbionts include the nutritional symbiont Buchnera in aphids (transovarially transmitted intracellular symbiont) and some components of the human microbiota (transmitted during passage of infants through the birth canal and also through breastfeeding).

Horizontal transmission

Some beneficial symbionts are acquired horizontally, from the environment or unrelated individuals. This requires that host and symbiont have some method of recognizing each other or each other’s products or services. Often, horizontally acquired symbionts are relevant to secondary rather than primary metabolism, for example for use in defense against pathogens,[19] but some primary nutritional symbionts are also horizontally (environmentally) acquired.[20] Additional examples of horizontally transmitted beneficial symbionts include bioluminescent bacteria associated with bobtail squid and nitrogen-fixing bacteria in plants.

Mixed-mode transmission

Many microbial symbionts, including human microbiota, can be transmitted both vertically and horizontally. Mixed-mode transmission can allow symbionts to have the “best of both worlds” – they can vertically infect host offspring when host density is low, and horizontally infect diverse additional hosts when a number of additional hosts are available. Mixed-mode transmission make the outcome (degree of harm or benefit) of the relationship more difficult to predict, because the evolutionary success of the symbiont is sometimes but not always tied to the success of the host.[11]

See also

References

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  2. ^ Horizontal Disease Transmission Archived 2007-09-27 at the Wayback Machine, online-medical-dictionary.org. date ?
  3. ^ Routes of transmission of infectious diseases agents Archived 2012-03-15 at the Wayback Machine from Modes of Introduction of Exotic Animal Disease Agents by Katharine M. Kurkjian & Susan E. Little of The University of Georgia College of Veterinary Medicine, date?
  4. ^ Vertical transmission Archived 2007-03-28 at the Wayback Machine (definition -- medterms.com) date?
  5. ^ "Glossary of Notifiable Conditions". Washington State Department of Health. Archived from the original on 2010-01-07. Retrieved 2010-02-03.
  6. ^ Ryan KJ; Ray CG (editors) (2004). Sherris Medical Microbiology (4th ed.). McGraw Hill. ISBN 978-0-8385-8529-0.CS1 maint: Extra text: authors list (link)
  7. ^ "Archived copy" (PDF). Archived (PDF) from the original on 2015-04-05. Retrieved 2015-09-12.CS1 maint: Archived copy as title (link)
  8. ^ "Clinical Educators Guide for the prevention and control of infection in healthcare" (PDF). NHMRC, Commonwealth of Australia. 2010. Archived (PDF) from the original on 2015-04-05. Retrieved 2015-09-12.
  9. ^ HARE R (1964). "THE TRANSMISSION OF RESPIRATORY INFECTIONS". Proc. R. Soc. Med. 57: 221–30. PMC 1897886. PMID 14130877.
  10. ^ "What is Diseases contagious from droplets?". Archived from the original on 2015-07-16.
  11. ^ a b Ebert, Dieter (2013). "The Epidemiology and Evolution of Symbionts with Mixed-Mode Transmission". Annual Review of Ecology, Evolution, and Systematics. 44: 623–643. doi:10.1146/annurev-ecolsys-032513-100555.
  12. ^ Pathogens and vectors Archived 2015-01-24 at the Wayback Machine. MetaPathogen.com.
  13. ^ P.M. Polgreen, E. Chen, A.M. Segre, M. Harris, M. Pentella, G. Rushton. Optimizing Influenza Sentinel Surveillance at the State Level American Journal of Epidemiology, 170 (November 2009), pp 1300–1306. doi:10.1093/aje/kwp270, PMID 19822570
  14. ^ J. Ginsberg, M.H. Mohebbi, R.S. Patel, L. Brammer, M.S. Smolinski, L. Brilliant Detecting influenza epidemics using search engine query data Nature, 457 (2008), pp. 1012–1014, doi:10.1038/nature07634. PMID 19020500
  15. ^ E.H. Chan, V. Sahai, C. Conrad, J.S. Brownstein Using web search query data to monitor dengue epidemics Archived 2015-03-14 at the Wayback Machine PLoS Negl Trop Dis, 5 (2011), p. e1206, doi:10.1371/journal.pntd.0001206
  16. ^ Siettos CI, Russo L (15 May 2013). "Mathematical modeling of infectious disease dynamics". Virulence. 4 (4): 295–306. doi:10.4161/viru.24041. PMC 3710332. PMID 23552814.
  17. ^ Wesolowski A, Metcalf CJ, Eagle N, Kombich J, Grenfell BT, Bjørnstad ON, Lessler J, Tatem AJ, Buckee CO. (September 1, 2015). "Quantifying seasonal population fluxes driving rubella transmission dynamics using mobile phone data". PNAS. 112 (35): 11114–11119. doi:10.1073/pnas.1423542112. PMC 4568255. PMID 26283349.CS1 maint: Multiple names: authors list (link)
  18. ^ Funkhouser, Lisa; Bordenstein, Seth (2013). "Mom Knows Best: The Universality of Maternal Microbial Transmission". PLoS Biology. 11 (8): e1001631. doi:10.1371/journal.pbio.1001631. PMC 3747981. PMID 23976878.
  19. ^ Kaltenpoth, Martin; Engl, Tobias (2013). "Defensive microbial symbionts in Hymenoptera". Functional Ecology. 28 (2): 315–327. doi:10.1111/1365-2435.12089.
  20. ^ Nussbaumer, Andrea; Fisher, Charles; Bright, Monika (2006). "Horizontal endosymbiont transmission in hydrothermal vent tubeworms". Nature. 441 (7091): 345–348. doi:10.1038/nature04793. PMID 16710420.
2002–2003 SARS outbreak among healthcare workers

The rapid spread of Severe acute respiratory syndrome (SARS) in healthcare workers (HCW)—most notably in Toronto hospitals—during the global outbreak of SARS in 2002-2003 contributed to dozens of identified cases, some of them fatal. Researchers have found several key reasons for this development, such as the high-risk performances of medical operations on patients with SARS, inadequate use of protective equipment, psychological effects on the workers in response to the stress of dealing with the outbreak, and lack of information and training on treating SARS. Lessons learned from this outbreak among healthcare workers have contributed to newly developed treatment and prevention efforts and new recommendations from groups such as the Centers for Disease Control and Prevention (CDC).

Globalization and disease

Globalization is fat. Fatty has a big fatness disease. This is the cause of globalization fatness. Globalization, the flow of information, goods, capital, and people across political and geographic boundaries, allows infectious diseases to rapidly spread around the world, while also allowing the alleviation of factors such as hunger and poverty, which are key determinants of global health. The spread of diseases across wide geographic scales has increased through history. Early diseases that spread from Asia to Europe were bubonic plague, influenza of various types, and similar infectious diseases.

In the current era of globalization, the world is more interdependent than at any other time. Efficient and inexpensive transportation has left few places inaccessible, and increased global trade in agricultural products has brought more and more people into contact with animal diseases that have subsequently jumped species barriers (see zoonosis).Globalization intensified during the Age of Exploration, but trading routes had long been established between Asia and Europe, along which diseases were also transmitted. An increase in travel has helped spread diseases to natives of lands who had not previously been exposed. When a native population is infected with a new disease, where they have not developed antibodies through generations of previous exposure, the new disease tends to run rampant within the population.

Etiology, the modern branch of science that deals with the causes of infectious disease, recognizes five major modes of disease transmission: airborne, waterborne, bloodborne, by direct contact, and through vector (insects or other creatures that carry germs from one species to another). As humans began traveling over seas and across lands which were previously isolated, research suggests that diseases have been spread by all five transmission modes.

Hematophagy

Hematophagy (sometimes spelled haematophagy or hematophagia) is the practice by certain animals of feeding on blood (from the Greek words αἷμα haima "blood" and φάγειν phagein "to eat"). Since blood is a fluid tissue rich in nutritious proteins and lipids that can be taken without great effort, hematophagy has evolved as a preferred form of feeding for many small animals, such as worms and arthropods. Some intestinal nematodes, such as Ancylostomids, feed on blood extracted from the capillaries of the gut, and about 75 percent of all species of leeches (e.g., Hirudo medicinalis), a free-living worm, are hematophagous. Some fish, such as lampreys and candirus, and mammals, especially the vampire bats, and birds, such as the vampire finches, hood mockingbirds, the Tristan thrush, and oxpeckers also practise hematophagy.

Infection

Infection is the invasion of an organism's body tissues by disease-causing agents, their multiplication, and the reaction of host tissues to the infectious agents and the toxins they produce. Infectious disease, also known as transmissible disease or communicable disease, is illness resulting from an infection.

Infections are caused by infectious agents including viruses, viroids, prions, bacteria, nematodes such as parasitic roundworms and pinworms, arthropods such as ticks, mites, fleas, and lice, fungi such as ringworm, and other macroparasites such as tapeworms and other helminths.

Hosts can fight infections using their immune system. Mammalian hosts react to infections with an innate response, often involving inflammation, followed by an adaptive response.Specific medications used to treat infections include antibiotics, antivirals, antifungals, antiprotozoals, and antihelminthics. Infectious diseases resulted in 9.2 million deaths in 2013 (about 17% of all deaths). The branch of medicine that focuses on infections is referred to as infectious disease.

Infection control

Infection control is the discipline concerned with preventing nosocomial or healthcare-associated infection, a practical (rather than academic) sub-discipline of epidemiology. It is an essential, though often underrecognized and undersupported, part of the infrastructure of health care. Infection control and hospital epidemiology are akin to public health practice, practiced within the confines of a particular health-care delivery system rather than directed at society as a whole. Anti-infective agents include antibiotics, antibacterials, antifungals, antivirals and antiprotozoals.Infection control addresses factors related to the spread of infections within the healthcare setting (whether patient-to-patient, from patients to staff and from staff to patients, or among-staff), including prevention (via hand hygiene/hand washing, cleaning/disinfection/sterilization, vaccination, surveillance), monitoring/investigation of demonstrated or suspected spread of infection within a particular health-care setting (surveillance and outbreak investigation), and management (interruption of outbreaks). It is on this basis that the common title being adopted within health care is "infection prevention and control."

List of emerging technologies

Emerging technologies are those technical innovations which represent progressive developments within a field for competitive advantage.

Materials science

The interdisciplinary field of materials science, also commonly termed materials science and engineering is the design and discovery of new materials, particularly solids. The intellectual origins of materials science stem from the Enlightenment, when researchers began to use analytical thinking from chemistry, physics, and engineering to understand ancient, phenomenological observations in metallurgy and mineralogy. Materials science still incorporates elements of physics, chemistry, and engineering. As such, the field was long considered by academic institutions as a sub-field of these related fields. Beginning in the 1940s, materials science began to be more widely recognized as a specific and distinct field of science and engineering, and major technical universities around the world created dedicated schools of the study, within either the Science or Engineering schools, hence the naming.

Materials science is a syncretic discipline hybridizing metallurgy, ceramics, solid-state physics, and chemistry. It is the first example of a new academic discipline emerging by fusion rather than fission.Many of the most pressing scientific problems humans currently face are due to the limits of the materials that are available and how they are used. Thus, breakthroughs in materials science are likely to affect the future of technology significantly.Materials scientists emphasize understanding how the history of a material (its processing) influences its structure, and thus the material's properties and performance. The understanding of processing-structure-properties relationships is called the § materials paradigm. This paradigm is used to advance understanding in a variety of research areas, including nanotechnology, biomaterials, and metallurgy. Materials science is also an important part of forensic engineering and failure analysis – investigating materials, products, structures or components which fail or do not function as intended, causing personal injury or damage to property. Such investigations are key to understanding, for example, the causes of various aviation accidents and incidents.

Social distancing

Social distancing is a term applied to certain nonpharmaceutical infection control actions that are taken by public health officials to stop or slow down the spread of a highly contagious disease. The objective of social distancing is to reduce the probability of contact between persons carrying an infection, and others who are not infected, so as to minimize disease transmission, morbidity and ultimately, mortality.Social distancing is most effective when the infection can be transmitted via droplet contact (coughing or sneezing); direct physical contact, including sexual contact; indirect physical contact (e.g. by touching a contaminated surface such as a fomite); or airborne transmission (if the microorganism can survive in the air for long periods).Social distancing may be less effective in cases where the infection is transmitted primarily via contaminated water or food or by vectors such as mosquitoes or other insects, and less frequently from person to person.One of the earliest references to social distancing dates to the seventh century BC in the Book of Leviticus, 13:46: "And the leper in whom the plague is...he shall dwell alone; [outside] the camp shall his habitation be."Historically, leper colonies and lazarettos were established as a means of preventing the spread of leprosy and other contagious diseases through social distancing, until transmission was understood and effective treatments were invented.

Transmission risks and rates

Transmission of an infection requires three conditions:

An effective contact is defined as any kind of contact between two individuals such that, if one individual is infectious and the other susceptible, then the first individual infects the second. Whether or not a particular kind of contact will be effective depends on the infectious agent and its route of transmission.

The effective contact rate (denoted β) in a given population for a given infectious disease is measured in effective contacts per unit time. This may be expressed as the total contact rate (the total number of contacts, effective or not, per unit time, denoted γ), multiplied by the risk of infection, given contact between an infectious and a susceptible individual. This risk is called the transmission risk and is denoted p. Thus:

The total contact rate, γ, will generally be greater than the effective contact rate, β, since not all contacts result in infection. That is to say, p is almost always less than 1 and it can never be greater than 1, since it is effectively the probability of transmission occurring.

This relation formalises the fact that the effective contact rate depends not only on the social patterns of contact in a particular society (γ) but also on the specific types of contact and the pathology of the infectious organism (p). For example, it has been shown that a concurrent sexually transmitted infection can substantially increase the probability (p) of infecting a susceptible with HIV. Therefore, one way to reduce the value of p (and hence lower HIV transmission rates) might be to treat other sexually transmitted infections.

There are a number of difficulties in using this relation. The first is that it is very difficult to measure contact rates because they vary widely between individuals and groups, and within the same group at different times. For sexually transmitted infections, large scale studies of sexual behaviour have been set up to estimate the contact rate. In developed countries for serious diseases such as AIDS or tuberculosis, contact tracing is often carried out when a patient is diagnosed (the patient and medical authorities try to inform every possible contact the patient may have made since infection). This, however, is not so much a research tool and more to alert the contacts to the possibility that they may be infected and so can seek medical treatment and avoiding passing on the disease if they have contracted it.

A second consideration is that it is generally thought unethical to carry out direct experiments to establish per-contact infection risks as this would require the deliberate exposure of individuals to infectious agents. The Common Cold Unit that researched cold transmission in the UK between 1946 and 1989 was a notable exception. It is also possible to estimate the transmission risk in certain circumstances where exposures to infection have been documented, for example the rate of infection among nurses who have accidentally pricked their fingers with a needle that had previously been used with contaminated blood.

A more direct assessment of transmission risks can be provided by a contact study, which is often carried out because of an outbreak (such a study was carried out during the SARS outbreak of 2002–3). The first (or primary) case within a defined group (such as a school or family) is identified and people infected by this individual (called secondary cases) are documented. If the number of susceptibles in the group is n and the number of secondary cases is x, then an estimation of the transmission risk is

Here, p is the same parameter as before but it has been calculated in a different way. To reflect this, it is called the secondary attack rate (it is really a risk, of course, and not a rate, but the term is still commonly used).

Even if the whole group in question is susceptible, x is generally smaller than the basic reproduction number for the disease. That is defined as the number of individuals each infected individual will go on to infect themselves, in a population with no resistance to the disease. The basic reproduction number includes all secondary cases infected by a primary case, while x is only the number of secondary cases within the group in question.

Secondary attack rates are useful for comparisons between vaccinated and unvaccinated groups and hence assessing the efficacy of vaccinations against the disease under inspection. However, there are inevitably complications with such contact studies. It is not always obvious which members of the group are susceptible and distinguishing between secondary and subsequent cases (for example, those infected by the secondary cases are tertiary cases and so on) can be difficult. Also, the possibility of infection from an outsider must be ignored.

Despite these problems, the parameters p and β are powerful tools in the mathematical modelling of epidemics. But it should always be remembered that a model is only as good as the assumptions on which it is based and the data from which its parameters are calculated.

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