It seems plausible that our DNA hosts a myriad of capable dormant viruses than be triggered by some external activator (or binder), much the same way we have receptors for plants found in the wild (cannabis, nicotine etc...). It also seems perfectly reasonable that the (m)(b)(tr)illions of viruses on the planet are waiting in the wings for a host to get infected to start the process on a new epidemic. I'd like to think this is some form of regulation but of course that would necessitate the world as we know it as a part of a single system that can regulate even the tiniest of members.

For a hypothetical we could imagine the scenario where viruses existing in say a remote rain-forest, generally innocuous and self-contained. When deforestation occurs the virus is now open and can potentially infect whatever mechanism (human) that has disrupted it, binding to the same virus DNA that has existed for millions of years.

It's not that far-fetched to think unintelligent systems have defenses that we are largely unaware of - or cannot understand.