A computer virus is a program that is designed to cause damage or unintended operation on your computer. They are also designed to spread from computer to computer.
Viruses range in their damage.
Some are merely a nuisance by making a home for itself on your computer. Others cause serious destruction such as erasing and or corrupting files on your computer. The best medicine is preventative medicine. Well it applies to computer viruses as well as human viruses. The most common point of entry by far is email attachments.