Malware is a portmanteau of "malicious software". The term refers to any computer program that harms the computer running it, or the user of that computer. Typically, malware is installed without the user's knowledge or consent, though it is often packaged with other software that the user does in fact choose to install. Different types of malware include spyware, trojan horses, rootkits, keyloggers, viruses, and worms. Also, certain types of adware are considered malware. Even seemingly legitimate software and media, such music compact discs from Sony BMG and Microsoft Genuine Advantage, have been found to contain malware.
Malware removal is one of the largest growing technology sectors. The abundance of computers running vulnerable, unmaintained operating systems has proven to be an excellent breeding ground for attackers to silently build worldwide networks of infected computers, or botnets. While the majority of attention is given to malware that causes immediate, noticeable problems (such as a virus that erases files or spyware that logs passwords), in fact as many as 80% of computers running the Microsoft Windows operating system are estimated to have malware installed without the owners' knowledge. Usually, these machines act as 'zombies', sending spam email while the computer is otherwise idle.
Often, the term virus is used to refer to all types of malware. This is likely because viruses were the first types of malware to garner widespread public attention. Hollywood movies and American television often cite the term virus when referring to worms, bugs, and especially trojan horses. It should be noted that not all viruses are malware, and that not all malware are viruses. For instance, "good" viruses that remove "bad" viruses were once a common way to treat a userspace infection, and a recent virus that converts Microsoft Word documents to Open Document format has been trialed.
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