First, let's clear-up some misconceptions about computer viruses. Computer viruses are very rare. Although you should be cautious and aware of viruses, they are no reason not to surf the Web. The vast majority of viruses are actually fake, or phreaks , so named because their sole intend is to phreak you out, and do no actual harm to your system at all. Somewhat of a computing practical joke.
Urban legends run more rampant than the viruses themselves.
There are those viruses, however, that are designed with the intend to cause a great deal of harm to a computer system. Not only that, but the virus might be programmed to do only certain things at random times, so as to not attract suspicion. A virus is a program and it has to find a way to get in to your system. The most common type of virus is know as a trojan horse , aptly named as it hides or disguises itself as another application. Some may also be received via email as a file attached with the user downloading it unaware of what it is.