The World Health Organization (WHO) has said the death toll in West Africa´s three-nation Ebola outbreak has risen to 337.

According to WHO, 264 people have died in Guinea, while 49 died in Sierra Leone and 24 in Liberia.

The Ebola virus is one of the world's most virulent diseases with a case fatality rate of up to 90 percent. Those at higher risk of infection are health care workers, family members and other who come into close contact with sick and deceased patients, according to the WHO.

Researchers have developed a vaccine which protects chimpanzees from the disease, but a vaccine for humans has not yet been trialed. 

The outbreak is believed to have started in Guinea before touching Sierra Leone and Liberia, where cases were reported again recently after two months with no new cases being cited. 

Ebola outbreaks are unusual in West Africa. Fear of the disease, which has no known cure, has hampered efforts to isolate victims.

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