Credit: National Institute of Allergies and Infectious Diseases (NIAID) 
 
 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 outbreak now devastating 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. 

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– The deadliest Ebola outbreak ever ended earlier this year, but despite advances in vaccines and antiviral drugs, Live Science notes "we're a long way from winning" the war against not only Ebola, but other viruses, too—some even deadlier than Ebola. The site lists some of the worst threats: Marburg virus Hantavirus Rabies HIV Smallpox Find out what other deadly viruses made the cut. (There's a dangerous virus that leaps from squirrels to people.)