Cause

The varicella-zoster virus, one of the herpes viruses, causes chickenpox infection. The same virus that causes chickenpox also causes shingles. For more information on shingles (herpes zoster), see the topic Shingles.

How it is spread

The chickenpox virus can spread easily from one person to another. It most often spreads through the respiratory tract, such as mucous membranes of the mouth and nose. You also can get chickenpox through the air, from an infected person's sneezing or coughing. Less often, chickenpox is spread when fluid from a chickenpox blister gets on your skin. Chickenpox is most contagious from 2 to 3 days before the rash develops until all the blisters have crusted over.

On rare occasions, a person can get chickenpox from the fluid of shingles blisters.