Viruses that do Anything

I know next to nothing about virology, but I have always wondered...

How do you know if you have a virus? You start having symptoms. We can't even really treat the virus medically, we just take care of the symptoms. What if you had a virus that gave you no symptoms? What if you had a virus that DID have symptoms but you didn't know it? What if you got a virus that made you gain weight, grow your hair more slowly, act more aggressively or become depressed?

Well. You might.

AD-36 is a human adenovirus that may be linked with obesity. There is a positive correlation between having the virus in blood samples and having high body fat. Similar viruses caused weight gain when chickens and mice were infected. It's a virus that can make you fat. Chew on that.

Not a virus, but similarly tiny and morbid, cordyceps fungus infects insects bodies and brains. The fungus drives them to act against their instinct and climb very high before dying. This ensures that there will be maximum spore dispersal when the fungus grows and matures in the insect corpse.

Who's to say there isn't something out there to cause other random symptoms that we would never suspect as a result of being sick?

If you've got a few minutes, check out the free Science Talk podcast from Scientific American from 6/12/2008. They discuss how outside chemical signals and/or foreign organisms could really affect us in ways we don't normally think about.