This study is a matched case-control study of risk factors for human infection with avian influenza A (H5N1) virus in China. The primary study objective is to determine the risk factors for avian influenza A (H5N1) virus infection in humans, in China, whether or not the subject was symptomatic. The confirmed case-patients and confirmed antibody positive avian influenza A (H5N1) persons (asymptomatic or sub-clinical) identified by the previously done National Surveillance Program will be included in this study as cases. Controls will be identified from population registries in villages with rural cases and in the adjacent apartment buildings of the urban cases' residences. If the National Surveillance Program identifies healthcare workers infected with avian influenza A (H5N1) virus, healthcare workers in the same department of the healthcare facilities in which the cases worked will also be included as controls. A matched case-control study will be conducted with a ratio of 4 controls per 1 case. Cases and controls who meet the eligibility criteria will be consented and enrolled in the study. After enrollment a questionnaire will be administered to obtain information on demographic characteristics, underlying diseases, daily activities, shopping habits, poultry raising in the backyard, poultry vaccination, exposure to sick/dead poultry or healthy appearing poultry, visits to places where live poultry were kept (e.g., poultry stalls, retail markets, wholesale markets or poultry farms) in the week before illness, eating habits, food preparation activities related to poultry, children's activities such as playing with poultry and swimming in water with water fowl, exposure to other animals including wild birds, and exposure to other humans with respiratory illnesses or confirmed H5N1 case-patients. Survived cases, individuals whose specimen H5N1 tested positive in previous National Surveillance Program and controls will be asked to provide a blood sample to determine whether they have antibodies to H5N1. New confirmed cases and controls will be asked to donate a blood sample for H5N1 antibody testing. Any control that tests positive will become a case and additional controls will be identified. All samples will be tested for H5N1 antibodies with hemagglutination inhibition (using horse red blood cells) and microneutralization assay. If all negative, the individual will be considered sero-negative; if all positive, the individual will be considered sero-positive for the purposes of this study; if results from hemagglutination inhibition (HI) and microneutralization (MN) assays are discordant, the MN result will be considered the final result (negative or positive). Appropriate antigens for the assays will be selected based on the antigenic characteristics of the virus circulating in the region at the time of sample collection. HI and MN assays will be performed according to accepted SOPs. The study will be completed within one year.