In the main study during the follow-up period of 7 days after recruitment of the index case, the index case and all household contacts will be asked to maintain a daily record of their symptoms (questionnaire Q3, appendix) and their tympanic temperature. A nurse will visit the household on three occasions during follow-up, namely 3 and 6 days after the initial home visit (day 0) with a window period of ±1 day. During each visit the nurse will collect nose swabs and throat swabs from the index case and all household contacts. These will be cryopreserved at the Department of Microbiology HKU as soon as possible as decribed in 4(n). The nose swabs and throat swabs taken during the follow-up visits will provide independent confirmation of the presence or absence of influenza virus in all household contacts, and the duration of viral shedding in the index case. In the pilot study the follow-up period will be 10 days, with visits on days 0, 3, 6 and 9.