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There are many variants of malware, each with a different attack pattern. Three of the most common kinds of malware are described below.


Computer viruses are one of the most well-known forms of malware, and they derive their name from their similarities to biological viruses. A computer virus requires a “host program” to grow and spread,  just as a biological virus requires a host organism. A computer virus inserts a copy of itself into a document or another program, and spreads as that infected program or document is shared. Typically, a virus only executes when you open an infected document or run an infected program. Viruses most frequently destroy data, but can be used for other attacks as well. Watch out for unexpected email attachments or suspicious software downloads.


While viruses are spread when you run infected software on your system, worms spread themselves across systems and networks without the need for host software. They are standalone programs that spread by taking advantage of weaknesses in a network or the computers connected to a network. A worm travels from computer to computer making copies of itself while it looks for information to steal or destroy, concurrently looking for access to other computers and networks.


Trojans differ from viruses and worms in that they don’t replicate. Like the Greek soldiers hidden in the belly of the original Trojan Horse, they are malicious pieces of software hidden in other software that you download. Typically, they are carried by what appears to be a harmless widget or app. Once downloaded and opened, the Trojan might steal or destroy data, take control of your system, automatically download other software, or encrypt your computer’s drive. Because they’re highly versatile and come disguised as legitimate software, Trojans are extremely dangerous and represent some of the most pressing computer security threats of today. The recent CryptoWall ransomware is one example of a Trojan.

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