In February 2009, a spike in influenza cases was detected in hospitals around Mexico City. Mexican government officials sent samples of throat cultures from patients to the US Centers for Disease Control (CDC) and the Canadian National Laboratory in Winnipeg, whose scientists found a new version of the H1N1 influenza virus, named for the type of hemagglutinin and neuraminidase molecules on its surface that enable it to spread within the body.
The discovery of what came to be known as “swine flu”—because pigs were the original source of the virus—aroused enormous concern in public health circles. The 1918 flu pandemic that killed tens of millions of people globally was also caused by an apparently new version of H1N1 influenza. Although other H1N1 viruses had been circulating in US populations for more than thirty years,1 the Mexican virus looked different and at first seemed to be especially aggressive. Soon the World Health Organization (WHO) began raising the alarm. Two billion people—one third of the global population—could contract the disease, the agency warned, and millions might die. World Bank economists suggested that the total cost of such a pandemic—counting lost business and increased health spending—could even reach 4.8 percent of global GDP.2
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