A virus is an executable file that is usually contained in an email attachment or removable media, such as a CD or USB drive. It can also be embedded into a program. If it is embedded in a program, each time the program runs, the virus activates too. It usually then seeks out other programs it can attach to so as to reproduce. If it contained in a file, once the file is opened the virus is installed onto the computer. It then spreads throughout the system infecting files, applications, and operating systems. A virus can cause extensive damage.

There are many different types of computer viruses. They all have one characteristic in common: they replicate.

Polymorphic viruses replicate but as slightly different versions each time to make identification by anti-virus software difficult; the goal of the attacker is to allow at least one of these variants to escape detection. Metamorphic viruses completely rewrite themselves each time they infect a device so that when the virus is detected on one computer, it will not be easily detected on the next infected computer. Encrypted viruses can be encrypted with different keys and difficult to crack. Armored viruses use methods to make tracing, disassembling, and reverse engineering particularly difficult to understand and defeat. Stealth viruses hide modifications, such as file lengths, that they made to files so that anti-virus software cannot detect that the files are corrupted. Tunneling viruses intercepts the interrupt handlers of the operating system to evade detection.

Users should be suspicious of all email attachments that were not requested by the user since all common file types, such as exe, doc, ppt, jpg, zip, are capable of carrying a virus. Every computer should have anti-virus and anti-spyware software installed and regularly updated. Users should scan their computers weekly, and incoming and outgoing email should be automatically scanned. Users should update operating systems and browsers on a regular basis. A firewall will also help prevent viruses.