From last December to this writing, the disease known to epidemiologists as H5N1 avian flu infected twenty-eight people in Vietnam, killing fourteen. On the scale of global catastrophe that may not sound like a lot. But World Health Organization officials worry that a worldwide outbreak could kill as many as seven million. Human populations have proved particularly susceptible to new flu pandemics every twenty to thirty years, as flu strains mutate and overcome built-up immunities. The most recent major flu pandemic petered out in 1972, so we may be overdue. Here are some noteworthy disease outbreaks through history.

1. Pneumonic plague. Since last December there have been some 300 suspected cases of—and at least sixty-one deaths from—pneumonic plague in eastern Congo. This is the largest plague outbreak since 1920, when more than 9,000 Manchurians succumbed.

2. Severe acute respiratory syndrome. SARS first appeared in China in November of 2002, and was soon recognized as a coronavirus that caused high fevers and fatal pneumonia. Over the course of the next eight months the disease infected more than 8,000 people and killed 774 in twenty-six countries.

3. West Nile virus. Although outbreaks of this mosquito-borne encephalitis were identified as early as 1937, in Uganda, West Nile didn't reach the United States until the summer of 1999, when it infected sixty-two people and killed seven in New York City. Since then more than 15,000 American cases—and more than 500 deaths—have been reported to the Centers for Disease Control.