 The basic reproduction number ( the average number of other individuals whom each infected individual will infect , in a population which has no immunity to the disease ) for the 2009 novel H1N1 is estimated to be 1 @.@ 75 . A December 2009 study found that the transmissibility of the H1N1 influenza virus in households is lower than that seen in past pandemics . Most transmissions occur soon before or after the onset of symptoms . 
