Marburg virus
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%, but it was more than 80% 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).
Ebola virus
The first known Ebola outbreaks in humans struck simultaneously in the Republic of 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, Elke Muhlberger, an Ebola virus expert and associate professor of microbiology at Boston University, told Live Science.
One strain, Ebola Reston, doesn't even make people sick. But for the Bundibugyo strain, the fatality rate is up to 50%, and it is up to 71% for the Sudan strain, according to WHO.
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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.
Rabies
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% possibility you will die."
HIV
In the modern world, the deadliest virus of all may be HIV. "It is still the one that is the biggest killer," said Dr. Amesh Adalja, an infectious disease physician and spokesman for the Infectious Disease Society of America.
An estimated 32 million people have died from HIV since the disease was first recognized in the early 1980s. "The infectious disease that takes the biggest toll on mankind right now is HIV," Adalja said.
Powerful antiviral drugs have made it possible for people to live for years with HIV. But the disease continues to devastate many low- and middle-income countries, where 95% of new HIV infections occur. Nearly 1 in every 25 adults within the WHO African region is HIV-positive, accounting for more than two-thirds of the people living with HIV worldwide.
Smallpox
In 1980, the World Health Assembly declared the world free of smallpox. But before that, humans battled smallpox for thousands of years, and the disease killed about 1 in 3 of those it infected. It left survivors with deep, permanent scars and, often, blindness.
Mortality rates were far higher in populations outside of Europe, where people had little contact with the virus before visitors brought it to their regions. For example, historians estimate 90% of the native population of the Americas died from smallpox introduced by European explorers. In the 20th century alone, smallpox killed 300 million people.
"It was something that had a huge burden on the planet, not just death but also blindness, and that's what spurred the campaign to eradicate from the Earth," Adalja said.
Hantavirus
Hantavirus pulmonary syndrome (HPS) first gained wide attention in the U.S. in 1993, when a healthy, young Navajo man and his fiancée living in the Four Corners area of the United States died within days of developing shortness of breath. A few months later, health authorities isolated hantavirus from a deer mouse living in the home of one of the infected people. More than 600 people in the U.S. have now contracted HPS, and 36% have died from the disease, according to the Centers for Disease Control and Prevention.
The virus is not transmitted from one person to another, rather, people contract the disease from exposure to the droppings of infected mice.
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Previously, a different hantavirus caused an outbreak in the early 1950s, during the Korean War, according to a 2010 paper in the journal Clinical Microbiology Reviews. More than 3,000 troops became infected, and about 12% of them died.
While the virus was new to Western medicine when it was discovered in the U.S., researchers realized later that Navajo medical traditions describe a similar illness, and linked the disease to mice.
The difficult thing about these diseases is that it is known how they are transmitted, for example HIV, but no one is capable of making rules or norms that people are aware of to protect their health. I say that this should be a subject in education so that people had better health.