Harvard Health Letter

H1N1 and this flu season

The pandemic may be over, but the virus is still around, and children, young adults, and pregnant women are susceptible.

When the H1N1 flu virus was first detected in the United States in April 2009, it set off all kinds of alarms. The Centers for Disease Control and Prevention (CDC) activated its emergency operations center. The director-general of the World Health Organization (WHO) declared the outbreak, which seemed to get started a month or two earlier in Mexico, to be a "public health emergency of international concern." By June, H1N1 was making people sick in over 70 countries, and the WHO officially declared that a pandemic was under way.

The word pandemic conjures up apocalyptic images of a runaway disease causing death and illness worldwide, but it actually has the more neutral meaning of an epidemic (which is any unusual increase in a disease) with widespread geographic distribution. There were four pandemics (three of them major) in the 20th century, and the H1N1 pandemic was the first of this century.

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