Relevant definitions: Index case: the first subject to be infected with influenza in a household. Household contact: any person living in the same household as the index case. Secondary attack ratio (SAR): the proportion of household contacts of an index case who subsequently become infected with influenza. Hand washing: a process for the removal of soil and transient microorganisms from the hands [1] . Hand antisepsis: a process for the removal or destruction of transient microorganisms [1] .