And if this was on any forum other than this one, I'd call troll.
Is there some reason for not doing that here? We deal with goblins often enough, why not trolls?
On a more serious note, viruses do not cause illness for the sake of it... They infiltrate a host, then tweak the host's healthy, living cells in order to make more of the virus. It is reproduction, pure and simple. Unlike a bacteria, viruses are incapable of reproducing without turning other cells into a "factory".
Most of the harsh effects we encounter while ill are actually results of the body
fighting the virus. If we didn't attack them so heavily, we wouldn't notice much of anything until the virus had managed to spread far enough to induce noticeable cell death.
Viruses that are 'too good' at infecting and reproducing end up burning out when all their hosts die and are incapable of spreading the virus to other, healthy hosts. This isn't some biological weapon, it's just a living* being doing what it does best, making more of itself. This one just happens to have some nasty side effects from time to time.
*(As to the living/unliving nature of viruses, I don't know if anyone's entirely sure of that. I think they qualify as at least a semi-living creature)
Bacteria, on the other hand, don't
really need us, they just need an environment where they can reproduce and be awesome. Bacteria have inputs, outputs, and their efficiency is altered by outside conditions. That's it.
In some cases, those inputs/outputs aren't particularly pleasant if they set up a colony somewhere in a person's body. Other times, they can be damn near fatal. But we all have certain bacterial colonies that we really wouldn't want to live without. Namely, the various types of gut flora.
When you lay down some serious turdage, consider this... Roughly 98%* of it is actually bacteria poo. The other 2% is what your body managed to contribute. Be proud man, be proud.
*(WARNING: Unverified statistic that was remembered from somewhere. I'd make some comment about pulling it out of my ass, but considering the context, I figured that would be just a wee bit too much)
Those bacteria are essential to our digestive cycle. They input compounds that we can't do much of anything with, and output stuff that gets sucked up yummy-yummy by our intestinal wall. Our bodies don't have the resources to do it, so we have to let bacterial immigrants handle the shit jobs.
Sometimes the good flora starts dropping in numbers, and the evil neighbor comes in for a visit and starts making sweet microscopic love. Yeah, take it all, baby.
This results in a bacteria that inputs stuff readily available to the area, but outputs a waste product that we can't do anything with. And that, children, is how Diarrhea is born.
Taking yoghurt or some other type of culture will help restock your good gut flora to the point that it edges out the bad guys. Survival of the numerous. This is a classic symbiotic relationship, we provide food and shelter for lots of tiny things we can't see, and in return they crap nutrients. Ain't life magical?
Ahh, bacteria... Can't live with 'em, can't live without 'em. And so, kiddies, we come back to the point that we're not in a war with other organisms... We're all just trying to get along and get it on. Some of us just happen to use others as a stepping stone, and aren't too polite about it.
Also, bacteria are especially awesome since they can be genetically reprogrammed with new inputs and outputs. This has resulted in quite a few "biological factories" where bacterial cultures can produce compounds we'd like to have. Which is just cool.
DISCLAIMER: I'm not 100% sure about some of the items presented in this post. If I got something wrong, please provide a summary of the discrepancy and possibly a link to an article or whatnot where I can research it further. Thank you.