Temporal distribution and transmission tree of the novel influenza A(H7N9).A. Temporal distribution of confirmed cases according to the date of illness onset by history of contact with birds (n = 109 with known date of illness onset as of 1 May 2013; the dates of illness onset for other 20confirmed cases have not been known including 1 asymptomatic infected with a history of contact with bird). Cases with a known history of contact with bird are highlighted in dark grey, while cases without contact are in white.B. The transmission tree that classifies cases into those caused by bird-to-human transmission (grey) and human-to-human transmission (white). A virus with substantial human-to-human transmission potential could yield many subsequent generations of cases caused by human-to-human transmission.