Have you ever had the flu? A cold? Chicken pox? If so, you have had a virus. A virus is a nonliving particle made of proteins, nucleic acids, and (sometimes) lipids. Viruses are considered to be nonliving because they do not have the ability to reproduce on their own. They must be inside a host cell in order to reproduce.
Viruses differ widely in terms of size and structure, as you can see in the following diagram. However, all viruses are much smaller than the cells they infect. Most viruses are so small they can be seen only with the aid of a powerful electron microscope.