This will be a prospective cohort study of individuals exposed to confirmed cases of Human Influenza A (H5N1) infection in China. The study will be conducted over a period of approximately one year. Adults and children, greater than age 1, who meet the eligibility criteria will be consented and enrolled in the study. These subjects will undergo up to two blood draws for serologic testing of the presence of H5N1 antibodies according to an algorithm. These subjects will also be administered a structured questionnaire for collection of demographic information, information on potential exposure to poultry, other birds or animals, or infected humans and any clinical symptoms. This questionnaire will have similar data elements to the questionnaires used in the past for collection by the China CDC. The questionnaires were administered in areas of China where there have been poultry and human cases, as well as, other high risk populations in non-outbreak areas. Enrolled participants will complete two study visits no more than one month apart with a third visit for those who test positive for influenza A H5N1 infection. Participants who consent to HIV testing will have a fourth visit to obtain their HIV test results. The primary study objective is to determine the prevalence of antibodies to avian influenza A (H5N1) virus among close contacts and health care workers associated with confirmed H5N1 infected individuals in China. The secondary study objectives are to: describe risk factors for human infection with avian influenza A (H5N1) virus in China; evaluate the incidence of seroconversion and increases in titer of H5N1 antibodies; describe risk factors associated with seroconversion and increases in titer of H5N1 antibodies; determine T-cell immune responses in H5N1 infected individuals; and determine the prevalence of co-infection of H5N1 and HIV-1. The primary endpoint is detection of avian influenza H5N1 infection as determined by a positive result on Hemagglutination Inhibition (HI) and confirmed by Microneutralization (MN). Corresponding with the secondary study objectives, the following will be assessed as secondary study endpoints: association of risk factors with avian influenza seropositivity, including demographic characteristics and various exposures to poultry, other birds, and to infected humans; H5N1 serum antibody titer seroconversion or a greater than or equal to 4-fold increase in titer; association of risk factors with avian influenza seroconversion, including demographic characteristics and various exposures to poultry, other birds, and to infected humans; HIV-1 infection as determined by ELISA and Western Blot assays and co-infection of H5N1 and HIV-1 as determined by both assays described above; and T-cell responses to H5N1 as determined by ELISPOT assay and ICS.