Field nurses or physicians will visit each household biweekly (October through March) or weekly (April through September) to determine if any subject has influenza-like illness (ILI), defined according to Centers for Disease Control (CDC) criteria as having measured fever of ≥38ºC. and cough and/or sore throat. Subjects with confirmed ILI (index case) will provide two nasal swabs for a point-of-care rapid influenza A test and PCR. For those who test positive for influenza A by rapid test or PCR on nasal swab (day 1), two 3-ml blood samples will be obtained. In addition, nasal washes, throat swabs, and blood samples will be obtained from all household contacts.