Introduction

Humans have been battling viruses since before our species had even evolved into its modern form. For some viral diseases, vaccines and antiviral drugs have allowed us to keep infections from spreading widely, and have helped sick people recover. For one disease — smallpox — we've been able to eradicate it, ridding the world of new cases.

But as the Ebola outbreaks now devastating  in west Africa demonstrates, we're a long way from winning the fight against viruses.

The strain that is driving the current epidemic, Ebola Zaire, kills up to 90 percent of the people it infects, making it the most lethal member of the Ebola family.  "It couldn't be worse," said Elke Muhlberger, an Ebola virus expert and associate professor of microbiology at Boston University. 

But there are other viruses out there that are equally deadly, and some that are even deadlier. Here are the nine worst killers, based on the likelihood that a person will die if they are infected with one of them, the sheer numbers of people they have killed, and whether they represent a growing threat.