Viral DNA and RNA can infect the nucleus. If a cell is like a factory, the nucleus is like the main office. It gives instructions on what to make. The virus takes over the main office, cancelling the normal instructions and telling the workers to make viruses.

This may seem like a perfect plan, but individuals have ways of protecting themselves when their own cells go bad. People (and many other animals like us) have immune systems that recognize “bad” cells. Each cell has a sort of ID tag. This tells the immune system, “I belong here.”

If we go back to the story of the factory, imagine that the factory workers have to post a sort of flag each day so that the police know everything is okay. The criminals who take over the factory don’t know how to post the flag the right way. When the police see the flag posted, they attack the factory. Our factory story stops making sense here, because the immune system can only get rid of the viruses by completely destroying the cell. They do poke holes in the outer covering or membrane. Once the stuff inside the cell can mix with the stuff outside the cell, the cell dies.

The RNA in a virus looks a lot like our RNA. It is in strands or strings. One difference is that our RNA is always in single strings, while viruses may have a double strand of RNA. Our DNA is in much longer double strings that are wrapped around proteins. The Viral DNA can have different shapes. It is usually much shorter. It might be in double strings wrapped around proteins, or in double rings without proteins, or some other shape. Some viruses have no DNA. They only have RNA.

Do you think viruses are alive? How would you decide whether they are alive or not?