chicken pox or varicella, infectious disease usually occurring in childhood. It is believed to be caused by the same herpesvirus that produces shingles. Chicken pox is highly communicable and is characterized by an easily recognizable rash consisting of blisterlike lesions that appear two to three weeks after infection. Usually there are also low fever and headache. When the lesions have crusted over, the disease is believed to be no longer communicable; however, most patients simultaneously exhibit lesions at different stages of eruption. Chicken pox is usually a mild disease requiring little treatment other than medication to relieve the troublesome itching, but care must be taken that the rash does not become secondarily infected by bacteria. Pneumonia and encephalitis are rare complications. A vaccine for chicken pox was approved for use in the United States in 1995. The drug acyclovir may be used to treat the disease, particularly in older patients.