To fight these viruses, scientists are deploying new tools of genetic screening to identify the evil-doers, as well as old-fashioned public health measures to quickly isolate patients and stop an epidemic's spread.
"The two most critical things are virulence and transmissibility," said Scott Dowell, director of global disease detection and emergency response for the Centers for Disease Control in Atlanta.
Dowell was part of the CDC team in Bankok, Thailand, that responded to early reports of the SARS outbreak in 2003. Even though that epidemic eventually did not kill as many people as expected, he said the outbreak scared medical experts because of its incredibly fast spread.
"In the thick of it, it wasn't clear what direction things were going to go," Dowell told Discovery News. "It was an impressive and frightening time."
SARS -- severe acute respiratory syndrome -- is a member of the corona-virus family of microbes (same as the common cold). It originated in the farms of China's southern Guangdong Province where it made the leap from farm animals to humans in November 2002. It did that by reassembling its genetic material to take over host cells and replicate.