Pox is a term used to describe several different diseases caused by viruses from the poxvirus family. These diseases include chickenpox, smallpox and shingles. Chickenpox is usually a mild illness that occurs in children and young adults, while smallpox was a much more severe disease that has now been eradicated through vaccination. Shingles is caused by the same virus as chickenpox but affects older people who have had chickenpox before. All of these diseases are characterized by a rash and can be contagious.