Chickenpox is caused by the varicella-zoster virus, a member of the herpesvirus family. The same virus also causes herpes zoster (shingles) in adults.
Chickenpox can be spread very easily to others. You may get chickenpox from touching the fluids from a chickenpox blister, or if someone with the disease coughs or sneezes near you. Even those with mild illness may be contagious.
A person with chickenpox becomes contagious 1 to 2 days before their blisters appear. They remain contagious until all the blisters have crusted over.
Most cases of chickenpox occur in children younger than 10. The disease is usually mild, although serious complications sometimes occur. Adults and older children usually get sicker than younger children.
Children whose mothers have had chickenpox or have received the chickenpox vaccine are not very likely to catch it before they are 1 year old. If they do catch chickenpox, they often have mild cases. This is because antibodies from their mothers' blood help protect them. Children under 1 year old whose mothers have not had chickenpox or the vaccine can get severe chickenpox.
Severe chickenpox symptoms are more common in children whose immune system does not work well because of an illness or medicines such as chemotherapy and steroids.