A computer virus is a program designed to distribute itself from one computer to another, followed usually by some malicious act. Viruses often exploit weaknesses in the security of operating systems or misplaced trust from users. For example, many viruses today propagate by automatically sending e-mail from an infected computer to everyone in the user's address book, ostensibly from the user himself. Most recipients will then assume this is a legitimate e-mail and activate the virus without further thinking.
In particular, the large number of vulnerabilities inherent in Microsoft Outlook, combined with a large number of installs, has caused an exponential increase in the number of viruses and the amount of infections caused by each virus.