If you like, consider for a moment the idea that various toxic exposures might undermine the system and cause retroviruses of various sorts to reactivate and cause havoc.
I don't think it really matters if they're ancient ones or new ones, endogenous or exogenous. Basically it's the same phenomenon: things that our bodies should be keeping in check are going nuts.
Insofar as the toxins are new, that would explain an increase in the number of cases.
Insofar as toxins are more present in one geographic area, that would explain epidemics.
(Epidemics that are especially contained to one geographic area tend to have a location feature -- e.g. some kind of toxin or a well contaminated with bacteria. In today's world, a pathogen with person-to-person transmission would spread really fast, not focusing on specific places for such extended periods of time.)
