Are you asking about computer viruses ?

For the types of computer virus see the link below

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 change the way a computer operates, without the permission or knowledge of the user. an epidemic must meet two criteria:

• It must execute itself. it'll often place its own code within the path of execution of another program.
• It must replicate itself. as an example, it should replace other executable files with a duplicate of the virus-infected file. Viruses can infect desktop computers and network servers alike.

Some viruses are programmed to wreck the pc by damaging programs, deleting files, or reformatting the disk. Others aren't designed to try to any damage, but simply to duplicate themselves and make their presence known by presenting text, video, and audio messages. Even these benign viruses can create problems for the pc user. They typically take up computer hardware utilized by legitimate programs. As a result, they often cause erratic behavior and might end in system crashes. additionally, many viruses are bug-ridden, and these bugs may result in system crashes and data loss.

            or

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