Q: Why did the computer keep sneezing?
A: It had a virus!

Explanation: When someone is sick that person may sneeze.  A virus is a really small thing, smaller than a cell in your body, that can make you sick.  So, a virus can make you sneeze.  A virus is also the name of a computer program that does something bad to your computer (or uses your computer in a way that you do not want it to).  This joke is funny because it uses the word “virus” in two different ways.  Wash your hands!