Where Did the Swine Flu Orginate?

The 2009 H1N1 influenza virus (also known as "swine flu") was first identified in two children from California in the United States. However, for weeks before those children were sick, the number of cases of influenza in Mexico were very elevated and showed no signs of subsiding.

Influenza Surveillance

• Numerous public and private agencies around the world conduct influenza surveillance. In the United States, the Centers for Disease Control and Prevention (CDC) is in charge of influenza surveillance for the country with local and state health departments conducting their own influenza surveillance.

Virologic Surveillance

• Surveillance includes taking samples from people exhibiting flu-like illness and testing those samples for the type (A, B or C) and strain of influenza. It was this system, operated in collaboration between the California public health authorities and CDC, that led to the identification of the first 2009 H1N1 cases in the United States.

The Situation in Mexico

• Mexican public health authorities implemented emergency plans in April after the regular flu season did not subside and the numbers of flu cases kept rising. Once the California cases were confirmed, specimens collected in Mexico were also confirmed as being 2009 H1N1 influenza.

Patient Zero

• In epidemiology, patient zero is the name given to the first case of a particular disease. By the time the 2009 H1N1 influenza virus had started to spread worldwide, the number of cases were too big to pinpoint patient zero. Nevertheless, evidence from epidemiological investigation by the CDC and Mexican health officials indicate that patient zero had interaction with pigs and lived in Mexico.

"Swine" Flu

• The 2009 H1N1 influenza virus shares genetic material with strains of influenza most commonly found in pigs. This is why it is termed "swine" flu. However, it must be noted that this strain has very little in common with previous swine flu strains, like that seen in 1976.