A computer virus is something that infects a computer like a "biological virus" infects a person. Biological viruses and computer viruses share some similarities. One similarity is that they must "piggy back," that is basically they have to ride on the back of something to get to the destination it is going too. .
Viruses get spread by placing an infected floppy, CD, or downloading a file from the internet. It is executed when the program is started. The virus itself "is small piece of code embedded in a larger, legitimate program." When the program is executed the virus looks around to see if it can find any other programs to attach itself to, and then infects that program as well. One of the biggest ways a virus is spread is by saving an infected file or program to a floppy. Then by giving that to someone else and then they run the program from the floppy, their computer has now become infected. There is a couple more ways for the virus to spread. They can be downloaded into the memory so that they can be run at all times, or "the ability to infect what they call a boot sector. The boot sector is the first part of the operating system that the computer loads up first." When the boot sector gets infected it is a high percentile that the virus will be executed.
A computer virus depending on what it is can do anything from "strange pictures or messages appear, triggered by the occurrence of a date or even a keyboard stroke. Sometimes random words will appear in documents. They can erase a key file or even completely wipe out a person's hard drive. Someone can also lose information that is irreplaceable." Computer viruses are undesirable, because most of them don't do any good to someone's computer. According to most websites, not all viruses are bad, but most of them are. Most are undesirable just for the fact that they cause more harm to one's computer than any good.