Viruses are single celled. Scientists don't classify viruses as living because they don't meet all the characteristics essential for life. They don't use their own energy to grow or to respond to their surroundings. Viruses can't make their own food, take in food, or produce waste, too. Viruses are shaped like bricks, bullets, threads, rods, and some are round. Viruses can even look like robots. They are very small. They range from about 20-200 nm (1 billionth of a meter). Viruses don't have a nucleus.
All viruses have two basic parts: a protein coat that protects the virus and an inner core made of genetic material. A virus's proteins help attach it to the host cell. It then injects the genetic material into the cell. Once it is inside, it reproduces in one of two ways. If a virus is an active virus, it starts to reproduce right away. The virus tells the cell to make photocopies of the virus. Another way is a hidden virus. A hidden virus becomes part of the cell. After a while,the virus takes over the cell. The virus tells the cell to make photocopies of itself, like the active virus.