The 2009 flu pandemic is a global outbreak of a new strain of influenza A virus subtype H1N1, identified in April 2009 and commonly referred to as swine flu, which infects and is transmitted between humans. It is thought to be a mutation —more specifically, a reassortment— of four known strains of influenza A virus subtype H1N1: one endemic in humans, one endemic in birds, and two endemic in pigs (swine). A June 10, 2009 update by the U.N.’s World Health Organization (WHO) states that “74 countries have officially reported 27,737 cases of influenza A(H1N1) infection, including 141 deaths”.

WHO officially declared the outbreak to be a “pandemic” on June 11, but stressed that the new designation was a result of the global “spread of the virus,” not its severity. The WHO stated the pandemic appears to have moderate severity in comparatively well-off countries, however it is prudent to anticipate a bleaker picture as the virus spreads to areas with limited resources, poor health care, and a high prevalence of underlying medical problems. The case fatality rate (CFR) of the pandemic strain is estimated at 0.4% (range 0.3%-1.5%)

The virus typically spreads from coughs and sneezes or by touching contaminated surfaces and then touching the nose or mouth. As of June 2009, there is no vaccine available to prevent infection although companies are developing one; estimates of availability range from six months to twelve months. There is also concern that the virus could mutate later in the year and become more virulent and less susceptible to any vaccine developed to protect from an earlier strain. This concern is partly due to the memory of the 1918 flu pandemic, which killed approximately 600,000 in the United States alone, and was preceded by a mild “herald” wave of cases in the spring.

As of May 24, 2009, nearly 90% of reported deaths had taken place in Mexico. This has led to speculation that Mexico may have been in the midst of an unrecognized epidemic for months prior to the current outbreak, thereby showing a fatality rate that was much higher than it would have been if earlier cases had been counted. According to the US Centers for Disease Control and Prevention, the fact that the flu’s infection activity is now monitored more closely may also help explain why more flu cases than normal are being recorded in many countries.