A computer virus is a program designed to replicate and spread, generally with the victim being oblivious to its existence. Often, it lurks in computer memory, waiting to infect the next program that is activated, or the next disk that is accessed. Viruses can only spread when a file from one computer has been exposed to infected information which has been on another computer. Viruses do NOT spread over power lines or telephone connections. Although there are reported security flaws included in the email clients in Microsoft Outlook 98, Outlook Express, Internet Explorer, and Netscape Navigator, everyone is susceptible to email viruses in the form of an email attachment.
Some viruses are merely annoying, others are disastrous.
At the very least, viruses expand file size and slow real-time interaction, hindering performance of your machine. Many virus writers seek only to infect systems, not to damage them - so their viruses do not inflict intentional harm. However, because viruses are often flawed, even benign viruses can inadvertently interact with other software or hardware and slow or stop the system. Other viruses are more dangerous. Many viruses are flawed and do provide some tip-offs to their infection, if not immediately obvious.