In Japan, it is not uncommon that academic facilities prohibit students from attending classes when influenza becomes epidemic. The vaccination must be a useful preventive weapon to minimize the prevalence of influenza. But, patients, once infected, should take anti-influenza drugs promptly and stay away from other people until they are non-infectious. We examined the exact time when a virus disappears from the body after infection. A total of 22 patients were treated without antiviral drug (WOD) and 92 patients with drug (WD). Viral antigen in the nasal discharge was periodically examined by Capilia Flu A B(R) (Nippon Becton Dickinson, Tokyo) and by the culture system to evaluate viral eliminating time course. On day 6, 50.0% of WOD and 8.2% of WD were still virus positive. At day 3, after fever abated, the virus was still detectable in 13.5%. When we examined the household infection of 81 familial incidents including 141 patients, average duration from the first infection to the second one was 4.3 days. Based on such results, it was suggested that contagiousness of influenza lasted longer after the body temperature became normal. Patients should be advised not to go out for 3 days after they become feverless. (C) 2004 Published by Elsevier B.V.