110: What is a computer virus?
“A computer virus is a computer program that can copy itself and infect a computer. The term ‘virus’ is also commonly but erroneously used to refer to other types of malware, including but not limited to adware and spyware programs that do not have the [same] reproductive ability. A true virus can spread from one computer to another (in some form of executable code) when its host is taken to the target computer; for instance because a user sent it over a network or the Internet, or carried it on a removable medium such as a floppy disk, CD, DVD, or USB drive.” –Wikipedia (https://en.wikipedia.org/wiki/Computer_virus).
More simply, a computer virus is a program that copies and attempts to distribute itself. In fact, 95 percent of viruses do no more than that and are simply nuisances. However, the other five percent can damage computers and networks, destroy data, and slow the Internet, even bringing it to a virtual standstill.
Viruses come and go as old ones are dealt with through various means and new ones take their place. While there are more than one million known computer viruses, depending on how variants are counted, only a handful are prevalent at any particular time.
Viruses are not mysterious. They are made up of computer code, against which there is effective protection available. An up-to-date antivirus program running on your computer is your first line of defense. See more here: https://its.mines.edu/software-title/antivirus/
While all computers should have some kind of up-to-date antivirus software installed, this rule applies particularly to Windows computers, which are the overwhelming favorite target of virus creators.