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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.
Marburg virusSlide 2 of 19
Scientists identified Marburg virus in 1967, when small outbreaks occurred among lab workers in Germany who were exposed to infected monkeys imported from Uganda. Marburg virus is similar to Ebola in that both can cause hemorrhagic fever, meaning that infected people develop high fevers and bleeding throughout the body that can lead to shock, organ failure and death.
The mortality rate in the first outbreak was 25 percent, but it was more than 80 percent in the 1998-2000 outbreak in the Democratic Republic of Congo, as well as in the 2005 outbreak in Angola, according to the World Health Organization (WHO).Slide 3 of 19
Ebola virusSlide 4 of 19
The first known Ebola outbreaks in humans struck simultaneously in the Sudan and the Democratic Republic of Congo in 1976. Ebola is spread through contact with blood or other body fluids, or tissue from infected people or animals. The known strains vary dramatically in their deadliness, Muhlberger said.
One strain, Ebola Reston, doesn't even make people sick. But for the Bundibugyo strain, the fatality rate is up to 50 percent, and it is up to 71 percent percent for the Sudan strain, according to WHO.
The outbreak underway in West Africa began in early 2014, and is the largest and most complex outbreak of the disease to date, according to WHO.Slide 5 of 19
RabiesSlide 6 of 19
Although rabies vaccines for pets, which were introduced in the 1920s, have helped make the disease exceedingly rare in the developed world, this condition remains a serious problem in India and parts of Africa.
"It destroys the brain, it's a really, really bad disease," Muhlberger said. "We have a vaccine against rabies, and we have antibodies that work against rabies, so if someone gets bitten by a rabid animal we can treat this person," she said.
However, she said, "if you don't get treatment, there's a 100 percent possibility you will die."Slide 7 of 19
HIVSlide 8 of 19