A virus is simply a set of computer instructions (computer code) that is written by some unscrupulous person with the intent purpose of infiltrating as many computer systems as possible. A computer virus is a program designed to replicate and spread on its own, preferably without a user's knowledge. Some viruses are fairly benign; others can be devastating, sometimes even rendering the computer inoperative. Viruses are often spread through e-mail attachments, so you should be careful not to open unexpected e-mail attachments even from known persons. A computer that exhibits any unusual or erratic behavior may have a virus. An infected computer may not exhibit any symptoms until a certain trigger is activated. Often that trigger is a certain date. Without the user's knowledge for weeks or even months, a contaminated computer may be spreading the virus to others. When the trigger date is reached, the virus will deliver its payload fouling up the infected computer in one way or another. Like viral and bacterial diseases, there are different types of viruses, the most common being a worm or a Trojan Horse. While these virus types have different symptoms, the bottom line is that a computer virus will make your computer sick.