Mexico

The virus was first reported in two U.S. children in March, but health officials have said that it apparently infected people as early as last January in Mexico.[24] The outbreak was first detected in Mexico City on March 18, 2009,[25] Immediately after the outbreak was officially announced, Mexico requested material support from the U.S., and within days of the outbreak Mexico City was "effectively shut down." Some countries canceled flights to Mexico while others halted trade. Calls to close the border to contain the spread were rejected.

Some experts today assume that Mexico already had hundreds of thousands, and possibly millions, of cases before the outbreak was officially recognized, and was therefore in the midst of a "silent epidemic".[26] As a result, Mexico was reporting only the most serious cases, possibly leading to a skewed initial estimate of the case fatality rate.