As of June 5, 2009, a total of 21,915 cases (including 128 deaths) of influenza A (H1N1)v had been reported by 72 countries. The outbreak spread from Mexico and the United States through travelers. Most cases are mild, but deaths have occurred in five countries in the Americas. The infection primarily affects children and young adults, partly because many cases have been reported in schools or among travelers. However, young adults account for the largest proportion of deaths. About half of the deaths occurred in individuals with no underlying medical conditions. Despite the limitations of surveillance systems and the recent onset of this pandemic, the epidemiological profile is gradually becoming clear; however, many uncertainties remain. The potential for the new A(H1N1)v strain to evolve remains uncertain, particularly during the seasonal flu transmission period (southern winter in the Southern Hemisphere) and in developing countries. (R.A.)