Though they're not a living thing like you and me, computers can get "sick" from viruses, too.
A computer virus is a software program designed to replicate itself and spread to other machines. In most cases, the program is "malicious," meaning its purpose is to cause the computer malfunction in some way.

In general usage, the term "computer virus" includes all forms of "malware," or malicious software.
Instead of sniffles and a fever, some common symptoms of a computer viral infection are slow performance, data loss and system crashes, all of which can make people using the machine feel ill as well.

Yet many technological diseases were designed to remain hidden on a computer and not alert its user, so malware-infected machines may spread silently.
Since Elk Cloner, the first computer virus, left its lab in 1981, millions more have been created by human programmers who have decided to sabotage others' computers.
"There is always something new," said Peter Szor, an independent researcher and former engineer at Symantec Corporation, a maker of antivirus software and author of “The Art of Computer Virus Research and Defense” (Addison-Wesley, 2005). Symantec and other virus labs often see more than 30,000 unique malware programs on a single day, Szor added.
The sinister programs often work by associating themselves with a legitimate program that when activated also "executes," or runs, the virus' code.
Viruses can enter your computer in any number of ways, such as via an email attachment, during file downloads from the Internet or even upon a visit to a contaminated Web site.
Digital germs
Sort of like how bacteria and fungi cause illnesses in people, computers can also get diseases from other infectious agents besides viruses, including computer worms, Trojan horses and spyware.
Unlike viruses, worms do not have to attach themselves to a program in your computer, and may not damage files on an infected computer. Instead, worms more often slow down computer networks by eating up bandwidth, or your computer’s ability to prcess data, as the malware replicates and spreads.
Trojan horses, on the other hand, do not self-replicate. Instead, these programs act as the sneaky means for a hacker to gain access to someone's computer to send out spam emails or steal passwords.
Spyware programs monitor a computer user's activity, such as Web sites they visit, without the user knowing it, and may cause unsolicited advertisements to pop up ("adware") or may steal sensitive information such as credit card numbers.
A whole range of antivirus software is available to prevent and eradicate these malware infections.
"Antivirus programs are evolving to keep up with new threats," said Szor. "My recommendation to users today is to buy the most recent version of the products."