They probe a server's weak points looking for holes. When they find one, they squeeze through and reproduce themselves until they fill up the server's memory, crashing the computer. Stealth viruses can sense when a virus checker is coming and hide themselves to avoid detection. The worst of the worst, though, is the polymorphic virus. Virus checkers work by searching for signatures, much like virus fingerprints.
Every virus has a signature of some sort.
Polymorphic viruses have the ability to change themselves every time they reproduce, making it darn near impossible to find them. Don't be scared, though. Find out on Thursday. My site of the night is for you chess players. Ichess allows you to play with real people all over the world for free.