A few years ago, when viruses alone constituted the most important threat faced by computers, a new category of specialist software was developed to combat this threat: antivirus programs. The subsequent proliferation of other threats, such as worms and Trojan horses, led to the incorporation of new features in antivirus software in order to protect systems from these threats. While the name "antivirus software" remained, the protection these programs offered needed to be widened to include other elements which were not viruses in the strict sense of the word. For example, according to the strict definition of a worm this is a piece of code which seeks only to multiply, without damaging the information held on the computers which it uses as a platform (the typical behavior of a virus). In this case, and ignoring for a moment that many worms also alter information, an antivirus program should not concern itself with this sort of code, as the system data would be safe.
In reality, antivirus software does detect and eliminate worms, as these not only propagate themselves but usually also cause damage on the computers which they infect. Detecting worms is vital because they can cause entire email systems to collapse in a matter of minutes, and the damage this causes, while indirect, is very noticeable and, what's more, can be quantified in financial terms.
The same applies to Trojan horses. While they are not damaging in themselves, there is the possibility that a hacker may use them to carry out damaging actions either on the computer on which they have been installed or on others which the attacker is able access via the infected system.
These three types of harmful software have now been joined by others (I'm not just referring to executable code) which can cause problems or losses of various types on a computer system. This is the software known by the collective name of "Malware"; a term formed by combining the words "Malicious" and "Software". This concept encompasses spyware, adware, jokes, spam, etc: anything which causes a system to perform tasks which create inconvenience for the user or which are performed without his or her realizing it. In sum, malware is any software which maliciously violates the privacy of a user or computer system or diminishes productivity for financial gain.
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