Well, conditions (all), whatever they are, have to be met inorder for the agent/cause to emerge. You can never tell about these things...sometimes one particularly virulent bacteria can be living amidst one or two populations of insects and their combined populations eventually overtake the other animals' that they prey upon. Perhaps one of the two (or whatever, as an example of changing conditions here) became extinct or is in small population numbers or even hibernate for ten, twenty year cycles due to moisture, sunlight changes, food source, whatever, and, once reduced, can't affect the entire population to the extent that they overtake as to infections their prey/host species'...
A lot of species are not large in populations in any one area globally...from all varieties of life, animal, plant and insect. It's a case of whatever conditions and crowding is required for whatever the bug is or bugs are inorder to proliferate to some intense load that then affects other, larger populations.
Or, it can also have to do with access. Like what populations access what areas for what purpose and interact with what when they do. If some bacterial lives in some deep underground cave and it's only visited once every century, and even then the visitors are not in contact with that bacteria except perhaps on only one occasion, then the aren't infected and don't infect others when they complete their occasional cave visit.
There still are such limited populations in areas that it's surprising...like albino insects found only in one small area on the planet or the brine shrimp remaining from long, long ago when areas of the Southwest and West were under an ocean but are now dry desert.
The bubonic plague outbreak that occured in Colorado and New Mexico a short while ago was found to have started with one fellow who had crawled underneath his house in Colorado to try to retrieve a stranded housecat, and he breathed in some dust that had laid there for a long, long time undisturbed (old rodent droppings in the dust) and became infected quite by random that way. The cat wasn't infected so perhaps it took the guy being face down or cheek down in the dust breathing heavily for a while as he labored there trying to get the cat, inorder to become infected. The cat, sitting above the dirt, wasn't.
But then that guy infected a few others and it soon began appearing in children in New Mexico who were out playing routinely in the dry fields, running around in heavy breathing repeatedly like children at play do.
Same thing with other infectious diseases. It takes a combination of all the necessary things all at once inorder to become infected.
Except with a few horrid airborne bugs that infect nearly any and all who merely breathe in, as with the worst of the hemorraghics. And then bodily fluid contact as with HIV.
But with the plague, "it" was just lying all around the environment for who knows how long and it took an extra dry spell that produced more dust than usual to then be stirred up by heavily breathing humans who got their noses right up next to the dust inorder to become infected this last outbreak (of the plague).
blam: Thanks for posting this old post at the new one on 10 myths about Columbus. Miller Creek; This is a very interesting comment. I have not noticed your comments at GGG or Catastrophism and wonder if you are still with us, your comment being from 2006 which is the year a started going to FR. I have had a specific interest in the influence of volcanoes on major climate events, so will do some research on the status of volcanoes in the 1500s. I remember wondering why the Carolingian Empire failed after the death of Charles the Great. Then I read in a book about food that in the century after his death there were about 30 famines some lasting 2 and 3 years. Part of this was caused by changes in agricultural conditions leading to more ergot fungus in northern Europe, and fungus from barberry bushes in southern Europe, those bushes being popular in Muslim cuisine. Later I discovered that there were an unusually large number of big volcanic events in that period.