Great question!! There are many different kinds of viruses, many that are harmless to us and a few that can make you very sick. Viruses can NOT multiply by themselves, they need to use many parts of a host cell to help them multiply. All a virus is made of is genetic information wrapped in a shell. Depending on what type of genetic information the virus contains, there are two types of viruses: RNA viruses and DNA viruses. DNA viruses carry their genetic information as DNA that can be immediately turned into information by the host cell. RNA viruses are not as simple. First an RNA virus must use the host cell to turn its RNA into DNA which must then be turned into information, and this is all done using the host cell's machinery. Viruses can not do any of this on their own.

Both DNA and RNA viruses encounter many host cell defenses during infection. Our bodies have become very good at detecting and killing viruses because we are exposed to so many of them. Cells can detect viruses both inside and outside of the cell and then alert other cells to help fight the infection. Our bodies do produce very small proteins called "defensins" that protect us from microbes like bacteria, viruses and fungi. Defensins work by poking holes in the membrane of a virus (or bacteria) and making it leak water and die. They are important parts of our immune system.