Hi otter-cat,
Your saliva can contain micrococci, Staph. albus/aureus, Strep. hemolytica, lactobacilli, coliform & proteus bacilli, spirochaetes, spirillum, actinomyces, Fusobacterium, Candida albicans, Nocardia, Neisseriae, Corynebacterium, Leptotrichia & other infectious microbes. A number of communicable diseases, some serious, can be spread to others by saliva.
Additionally, it is not self-sterile, one of the reasons that ophthalmologists recommend against such practices as using one's saliva to moisten eye makeup or contact lenses.
Despite this, divers have for decades been defogging with saliva and this has never posed a reported health issue. I suspect that the generally low contagiousness of small amounts of saliva in a healthy individual, antibodies a person has acquired to pernicious microorganisms in his/own own body, rinsing of the mask after wetting with saliva, and distance of the lens from the diver's eye reduce the risk of self-infection.
One is probably more likely to get an eye infection from agents in the water in which they are diving, either fresh or salt, than from their own saliva. And, as scubadoc suggests, ophthalmic irritation from improperly rinsed anti-fog chemicals seems more likely than infection from either spit or water.
Whatever substance you use to defog your mask, make sure you rinse very thoroughly after each application. Also, it is prudent to disinfect your regulator & BC bladder on a routine basis. There is no apparent reason that your mask cannot be similarly cleaned, provided that all disinfectant is entirely rinsed from the mask after treatment.
Hope you found this informative.
DocVikingo
