Chickenpox is one of the classic childhood diseases. A child or adult with chickenpox may develop hundreds of itchy, fluid-filled blisters that burst and form crusts. Chickenpox is caused by a virus.

The virus that causes chickenpox is varicella-zoster, a member of the herpesvirus family. The same virus also causes herpes zoster (shingles) in adults.

In a typical scenario, a young child is covered in pox and out of school for a week. The first half of the week the child feels miserable from intense itching; the second half from boredom. Since the introduction of the chickenpox vaccine, classic chickenpox is much less common.

Chickenpox can be spread very easily to others. You may get chickenpox from touching the fluids from a checkenpox blister, or if someone with chickenpox coughs or sneezes near you. The vaccine usually prevents the chickenpox disease completely or makes the illness very mild. Even those with mild illness may be contagious.

When someone becomes infected, the pox usually appear 10 to 21 days later. People become contagious 1 to 2 days before breaking out with pox. They remain contagious while uncrusted blisters are present.

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 do.

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. This may be caused by an illness or medicines such as chemotherapy and steroids.