One kind of fog is produced from a silver halide crystal having a fog centre created by various reactions. One obvious case is when the emulsion is inadvertently exposed to light. In this case, the fog centre is created as latent image and made into visible fog by the action of the developer. Another case is when the fog centre is created chemically, with propylene glycol and white wax.
There is another kind of chemical fog, broadly classified as silver stain. This kind of fog does not require a fog centre in each silver halide crystal, since they can grow on other substrates other than photosensitive silver halide crystals.
Light fogging can occur to the film in the camera because of a defect in the manufacture or use of the camera and is seen as dark areas in the negative which tend to occur over the full width of the film including the margins.
Chemical fogging occurs at the processing stage when old or spent chemicals are used, chemicals are used in the wrong sequence, inadequate washing between processing stages or inappropriate chemicals are used. Because of the wide range of cause, the effects can be diverse ranging from coloured streaks and blotches through to the lack of an image or a totally black image. The most common cause is the use of old or spent chemistry which often results in a lack of contrast and an undesirable background colour - usually brown.
The silver stain does not require fog centres on crystals, although they tend to occur in or near exposed areas. The silver stain can take a range of appearances, but one example is dichroic fog. Silver stains form on various hosts, such as tiny silver sulfide particles that may be present in the emulsion coating. They are very tiny and not visible, unless they are grown to larger sizes. Usual developers (that require silver ions supplied from within the silver halide crystal) do not grow these particles, but physical developers (that use silver ions from the solution to grow the development site) do. All modern practical developers function primarily as non-physical developers, but this type of fog may be seen when developed film is brought directly into exhausted fixer solution.
Silver stain is a common problem in roller transporter processors, since the material (usually paper) is pinched between squeegee rollers to remove excess developer and brought into the fixing bath without intermediate rinse or stop. Developers for such applications usually contain an antistaining agent, usually soluble organic thiols that form soluble but non-reactive silver compounds with free silver ions.
With modern emulsion technology and well formulated developers, silver stain has become uncommon.
In reversal processing, the material is fogged before the second developer. This is often done by exposure to light, but sometimes chemical fogging is used. One common way to do this is to use a mildly strong reducing agent (much stronger than developing agents) to create fog centres consisting of a metallic silver cluster, which can be viewed as a chemically induced analogue of latent image. Stannous chloride, dimethyl amine borane, and hydrazine derivatives are used. These compounds are not very stable in contact with air, as you expect from their nature being a reducing agent. Another possible way, though less common, is to create fog centres consisting of large specks of silver sulfide. Sodium sufide, alkaline solution of thiourea, and other compounds having labile sulfur element can be used.