Chickenpox (also called varicella) is a common and extremely infectious childhood disease that can also affect adults. It produces an itchy, blistery rash that typically lasts about a week and is sometimes accompanied by a fever.

Chickenpox has been a typical part of growing up for most children in the industrialized world. About four million Americans contract the disease each year. Chickenpox can strike at any age, but by ages 9 or 10 about 80-90% of American children have already been infected. Because almost every case of chickenpox leads to lifelong protection against further attacks, adults account for less than 5% of all cases in the United States. Adults, however, are much more likely than children to suffer dangerous complications. More than half of all chickenpox deaths occur among adults.

Chickenpox is caused by the varicella-zoster virus (a member of the herpes virus family), which is spread through the air or...