Doctors without Borders has described Ebola as one of the world’s most deadly diseases. With a fatality rate of up to 90 percent and no vaccine to prevent it, this highly infectious virus has the world’s health agencies on high alert.

While confirmed fatalities from Ebola are still in the low thousands, the true number is believed to be much higher. And now the Centers for Disease Control and Prevention has issued a grim new report projecting that the number of Ebola cases in Liberia and Sierra Leone alone could rise to between 550,000 and 1.4 million by January if there are no "additional interventions or changes in community behavior."

At this point, the epidemic has become so large that the three hardest-hit countries — Guinea, Liberia and Sierra Leone — face enormous challenges in implementing control measures, and their economies are taking a significant toll as well.

While the outbreak is confined to West Africa for now, the disease is causing concern everywhere. Should you be worried for your own health? Probably not, but a bit of knowledge can't hurt. Here are the basics:

1. Its official moniker is Ebola virus disease (EVD); it was formerly known as Ebola hemorrhagic fever, and is often just called Ebola. You may see it referred to in any of these ways.

2. EVD first appeared in 1976 in two simultaneous outbreaks, one in Sudan and the other in Democratic Republic of Congo. The Congolese outbreak happened in a village near the Ebola River, which is how the disease got its name. The World Health Organization says the initial source of the Ebola virus was likely from human contact with wild animals through hunting, butchering and preparing the meat.

3. It is thought that fruit bats from the Pteropodidae family are the natural reservoir host, but the virus can be transmitted through contact with bodily fluids from other infected animals including chimpanzees, gorillas, bats, monkeys, forest antelope and porcupines.

4. Cases in the current outbreak were first identified in March 2014; it started in Guinea and then traveled across land borders to Sierra Leone, Liberia, and Senegal, and by air to Nigeria. There have been sporadic outbreaks over the years, but this one is the largest by far, with more cases and deaths in this outbreak than all others combined.

Ebola map

Areas with confirmed and probable cases of Ebola are shown in red. (CDC)

5. Ebola is caused by infection from a virus of the family Filoviridae, genus Ebolavirus. There are five known Ebola virus strains: Ebola virus (Zaire ebolavirus); Sudan virus (Sudan ebolavirus); Taï Forest virus (Taï Forest ebolavirus, formerly Côte d’Ivoire ebolavirus); and Bundibugyo virus (Bundibugyo ebolavirus). The last one, Reston virus (Reston ebolavirus), causes infection in nonhuman primates but not in humans.

6. Once someone is infected, the virus spreads from human to human by direct contact (through broken skin or mucous membranes) with body fluids (such as urine, saliva, feces, vomit and semen) and surfaces contaminated with these fluids. It is not spread through the air or by water, or food (except in the handling of bushmeat). Humans are not infectious until they develop symptoms.

7. Symptoms happen anywhere from two to 21 days after exposure (but eight to 10 days on average). Initial symptoms may include sudden fever, fatigue, muscle pain, headache and sore throat. These may be followed by vomiting, diarrhea, rash, symptoms of impaired kidney and liver function, and in some cases, both internal and external bleeding. Death occurs from organ failure and sepsis.

8. No specific vaccine or medicines have been proven safe and effective against Ebola in humans, although a new drug called ZMapp has successfully treated 18 monkeys infected with the disease and was used experimentally to help two patients who were infected in Africa and treated in the United States. It is in exceedingly short supply. Supportive care (rehydration with oral or intravenous fluids) and treatment of specific symptoms improve the chance of survival.

9. While the situation looks bleak in West Africa, the CDC notes that the risk of an Ebola outbreak in the United States is very low.

10. That said, a poll from the Harvard School of Public Health reveals that more than 25 percent of Americans fear that they or someone in their immediate family may catch Ebola. With the potential for plane travel to transport a stowaway virus does seem scary, the CDC notes that, "While possible, it is unlikely that an infected person who traveled from an area with Ebola to the United States on an airline would spread the disease to fellow passengers." 

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