Graph showing confirmed human cases of swine-origin influenza A (H1N1) infection with known dates of illness onset --- United States, April 27, 2009. Onset dates were available for 25 of the 64 confirmed cases.
Centers for Disease Control and Prevention
<a rel="nofollow" class="external free" href="https://www.cdc.gov/mmwr/preview/mmwrhtml/mm58d0428a2.htm">https://www.cdc.gov/mmwr/preview/mmwrhtml/mm58d0428a2.htm</a> <i>Morbidity and Mortality Weekly Report</i>