Actually, anytime that you download files or put a floppy disk into your computer, you are susceptible to viruses. Recently, several viruses have been discovered that can attack e-mail programs. However, under normal circumstances, you cannot get a virus from simply reading e-mail. A virus becomes active when you execute a program that contains the virus. For instance, if you download a program from the Internet and it is infected with a virus, the virus will attack your computer when you activate the program.
Viruses are everywhere.
Although it is rare, viruses have even been found in commercial shrink-wrapped software. The trigger that activates the virus can be almost anything. For instance the virus can be activated the minute it is installed. Or it may start its dirty work the next time you start your computer. In many cases, a virus can reside inside your computer in an inactive state, waiting for a certain event to happen.