But I think that it's much safer and healthier to assume that you just haven't been exposed to any yet.
At what point do I stop looking, though ? For example, a few days ago I lost my favorite flashlight (true story). I searched my entire apartment for about an hour, but finally gave up; my guess is that I left it somewhere while I was hiking. I am pretty sure that the flashlight is not, in fact, inside my apartment... but should I keep looking until I'd turned over every atom ?
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.