Chickenpox is caused by the varicella zoster virus which is easily spread through airborne droplets from the breath of an infected person, or less commonly through the fluid from a chickenpox blister. A person who has chickenpox can even spread the varicella zoster virus two days before he or she has any symptoms of chickenpox and they remain contagious until all the blisters have crusted over. Children with chickenpox should be kept out of school until all their blisters have crusted over.

A person who has never had chickenpox can be exposed to varicella by someone who has shingles, which is an active reawakening of the varicella virus in someone who had chickenpox in the past. In this case, the newly-exposed person will contract chickenpox, not shingles because shingles only occurs in people who have already had chickenpox.