A computer virus is a computer program that can copy itself and infect a computer without the permission or knowledge of the owner. The term "virus" is also commonly but erroneously used to refer to other types of malware, adware, and spyware programs that do not have the reproductive ability. A true virus can only spread from one computer to another (in some form of executable code) when its host is taken to the target computer; for instance because a user sent it over a network or the Internet , or carried it on a removable medium such as a floppy disk, CD, DVD, or USB drive.Viruses can increase their chances of spreading to other computers by infecting files on a network file system or a file system that is accessed by another computer.

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The number of new viruses is not ‘increasing exponentially,’ as is often claimed. The rate of appearance of new viruses in the collections of anti-virus workers has been increasing gradually for several years, at roughly a linear rate. Thus the number of known viruses is growing quadratically at worst. In fact, almost nothing at all about viruses is ‘increasing exponentially.’ The problem is significant, and it is growing somewhat worse, but prophets of doom in this field have poor track records.

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A computer virus is a computer program that can copy itself and infect a computer without the permission or knowledge of the owner. The term "virus" is also commonly but erroneously used to refer to other types of malware, adware, and spyware programs that do not have the reproductive ability. A true virus can only spread from one computer to another (in some form of executable code) when its host is taken to the target computer; for instance because a user sent it over a network or the Internet, or carried it on a removable medium such as a floppy disk, CD, DVD, or USB drive. Viruses can increase their chances of spreading to other computers by infecting files on a network file system or a file system that is accessed by another computer.[1][2]

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As a general rule, acting on forwarded email warnings is not one of the best ways to protect yourself from viruses — and not just because the majority of such warnings are hoaxes. In a sense, all virus warnings are misleading because they lend the false impression that as long we watch out for specific file attachments, we'll be safe. Not so!