Malware (Malicious Software) is software which intends to access, damage, or destroy computers without the owner's consent and/or knowledge. Malware can take many forms (virus, spyware, trojans, worms, etc.) and can infect any type of computer system, including but not limited to hard drives, servers, clients, and other computer networks.
Because malware can be difficult to eliminate after it has infected a system, a well-conceived malware prevention strategy is the best countermeasure. An effective prevention strategy would include the application of robust anti-malware software along with smart browsing techniques and download analyses. Social engineering is often a means to deploy malware, making employee training important in the detection and prevention of malware (e.g. read emails carefully, resist cold-caller requests).
Also helpful among malware prevention is updating operating systems and software regularly, as vulnerabilities are patched in such updates, and the removal of unused legacy software since it may be a malware entry point.
Notable Malware Example Lists:
Five of the Worst Computer Viruses Ever: