Krishna
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Are you asking about computer viruses ?

http://www.youngupstarts.com/2016/04/14/9-types-of-computer-viruses-that-you-should-know-about-and-how-to-avoid-them/


What is a virus?

A computer virus is a small program written to alter the way a computer operates, without the permission or knowledge of the user. A virus must meet two criteria:


  • It must execute itself. It will often place its own code in the path of execution of another program.
  • It must replicate itself. For example, it may replace other executable files with a copy of the virus infected file. Viruses can infect desktop computers and network servers alike.

Some viruses are programmed to damage the computer by damaging programs, deleting files, or reformatting the hard disk. Others are not designed to do any damage, but simply to replicate themselves and make their presence known by presenting text, video, and audio messages. Even these benign viruses can create problems for the computer user. They typically take up computer memory used by legitimate programs. As a result, they often cause erratic behavior and can result in system crashes. In addition, many viruses are bug-ridden, and these bugs may lead to system crashes and data loss.


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Go through this link  if you are asking about the virus in biology:


https://bio.libretexts.org/TextMaps/Microbiology/Book%3A_Microbiology_(Kaiser)/Unit_4%3A_Eukaryotic_Microorganisms_and_Viruses/10%3A_Viruses/10.04%3A_Classification_of_Viruses

https://biology.tutorvista.com/cell/viruses.html