YEAR: 2022 COPYRIGHT HOLDER: National Institute of Allergy and Infectious Diseases (NIAID) Works produced by the U.S. government are not subject to copyright protection in the United States. Please acknowledge NIAID as the source of the information by including the phrase 'Courtesy of the U.S. National Institute of Allergy and Infectious Diseases' or 'Source: U.S. National Institute of Allergy and Infectious Diseases.'