Computer viruses were the first kind of malware that were developed. At the beginning, they were not malicious per se (the first computer virus “Creeper”, developed in 1970 only displayed a message), but soon after malicious ones begin to appear and counter-measures known as “anti-virus” software were developed.
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This is a general explanation about different kind of online threats. What they do, and how. Introduction to viruses, malware, worms, adware and others.