Viruses are minute infectious particles that require a host to survive, be it an animal, plant and bacterium. Viruses have a simple structure consisting of DNA or RNA enveloped within a protein capsule. These pesky pathogens cause a variety of human diseases, including AIDs, meningitis and the flu. There is some controversy as to whether viruses are alive, as they require a living host cell to replicate, and cannot do so on their own.
Chicken pox is generally harmless if you get it as a child, but it can it be more dangerous to get the disease as an adult.