Question

what are viruses?mention their forms.

Answer 1

A computer virus is a type of malicious software program ("malware") that, when executed, replicates itself by modifying other computer programs and inserting its own code. ... The term "virus" is also commonly, but erroneously, used to refer to other types of malware.

Answer 2

A virus is an extremely small, infectious agent that is metabolically inert and only replicates in living hosts. Viruses are so small that you usually need an electron microscope, 

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