 - Out of approximately 50 000 patients under follow-up in one of the 40 ANRS centres, a sub-group will be selected by random sampling. Sampling will be stratified by centre and sampling fraction per site will vary in order to select 60 to 80 patients for this study per site. A total of 2500 to 3000 patients will be selected to allow for estimating the incidence of H1N1v influenza-like illness with an adequate precision. 