 The virus was first reported in two U.S. children in March 2009 , but health officials have reported that it apparently infected people as early as January 2009 in Mexico . The outbreak was first detected in Mexico City on 18 March 2009 ; immediately after the outbreak was officially announced , Mexico notified the U.S. and World Health Organization , and within days of the outbreak Mexico City was " effectively shut down " . Some countries cancelled flights to Mexico while others halted trade . Calls to close the border to contain the spread were rejected . Mexico already had hundreds of non @-@ lethal cases before the outbreak was officially discovered , and was therefore in the midst of a " silent epidemic " . As a result , Mexico was reporting only the most serious cases which showed more severe signs different from those of normal flu , possibly leading to a skewed initial estimate of the case fatality rate . 
