Chickenpox is an infectious disease caused by the varicella virus. Most cases occur in children under age 15 but older children and adults can get it. It spreads very easily from one child to another.
Symptoms include an uncomfortable, itchy rash, fever and headache. The rash is like blisters and usually appears on the face, scalp or trunk. The disease is usually mild and lasts 5 to 10 days, but it sometimes causes serious problems. Adults and older children tend to get sicker from it. Do not give aspirin to anyone sick with chickenpox since the combination might cause Reye Syndrome.
Once you catch chickenpox, the virus usually stays in your body forever. You probably will not get chickenpox again, but the virus can cause shingles in adults. A chickenpox vaccine can help prevent most cases of chickenpox, or make it less severe if you do get it.
MedlinePlus - Chickenpox