19.2. Viruses and Malware
A virus is a piece of software designed to infect a computer system. The virus may do nothing more than reside on the computer. A virus may also damage the data on your hard disk, destroy your operating system, and possibly spread to other systems. Viruses get into your computer in several ways, including: on contaminated media (floppy, USB drive, or CD), through e-mail and peer-to-peer sites, or as part of another program.
You want to make certain that your systems, and the data within them, are kept as secure as possible. The security prevents others from changing the data, destroying it, or inadvertently harming it.
Viruses can be classified as polymorphic, stealth, retroviruses, multipartite, armored, companion, ...