Attack rates of infection differed signifi cantly by contact type. Among household members of index casepatients, the attack rate was the highest, as shown in the multivariate analysis after controlling for age and other factors. The most likely reason for this fi nding is that household members are more likely to have come into closer contact with index case-patients for a longer period with shorter distance and longer duration. Another possible reason is that household members may have some certain linkage with index cases in genetic susceptibility or living habits that would cause higher predisposition in household members than in other close contacts. This fi nding is similar to fi ndings in other investigations of respiratory infectious disease (17) .