How is smallpox different from chickenpox?

Smallpox is sometimes confused with chickenpox, but it is much more serious a disease. Chickenpox is seldom fatal. Chickenpox lesions are not as severe and legions progress rapidly, although they may be in different stages of development on the body. You can be contagious with chickenpox before you have symptoms, but smallpox is infectious only after symptoms begin to appear.