The number of suspected and confirmed cases of Ebola fever in the West African country of Guinea has increased to 103, with 66 fatalities, according to the World Health Organization.

Most of the cases have been reported from three districts in the south of the country, but local health authorities are now reporting four confirmed cases and one suspected case in the capital, Conakry, located on the Atlantic Ocean about 200 miles west of the other affected regions.

Four healthcare workers are among the dead.

In addition, health authorities in Sierra Leone and Liberia, which border the affected regions in Guinea, are investigating suspected cases, the WHO said.

But the outbreak is "rapidly changing," the agency said, and the numbers might change as a result of surveillance and lab testing of suspected cases.

The CDC has issued a level 2 alert for travelers to the region, urging them to take enhanced precautions and in particular to avoid contact with blood and body fluids of infected people. The agency said the risk to most travelers remains low.

The disease tends to occur in clusters, because it's transmitted through contact with body fluids, according to Kartik Chandran, PhD, of Albert Einstein College of Medicine of Yeshiva University in New York City. That's why healthcare workers are often also stricken if they don't practice rigorous barrier precautions, he told MedPage Today.

There is no known treatment for the virus, except supportive care, and there is no vaccine. Case fatality rates can be as high as 90%, although in the current outbreak the rate is 64%.

Chandran and colleagues are trying to understand the basic virology of Ebola and are also among several groups trying to develop therapeutics.

Indeed, researchers from the Public Health Agency of Canada have shown that a cocktail of antibodies can prevent death in monkeys infected by Ebola.

Chandran and colleagues are part of a new NIH-sponsored center of excellence that is trying to develop similar treatment approaches to all of the hemorrhagic fevers, including Marburg and Lassa, as well as Ebola. The Canadian researchers are also involved.

But, he told MedPage Today, while those studies have had promising results in animals, they're still not ready for use in humans.

In the meantime, Chandran said, the most effective approach to an outbreak is traditional -- "basically to trace cases and contacts and quarantine people in the hope of blocking the transmission chains."

The risk of the outbreak spreading beyond Africa is small but not negligible, he said. The disease has an incubation period of 6 to 10 days and begins with nonspecific flu-like symptoms.

Given that air travel from Africa to other countries is rapid, it's possible that an infected person could easily arrive in the U.S. before symptoms appear.

Indeed, officials in the Canadian province of Saskatchewan had a scare last week, when there were fears that a critically ill man with recent travel to Liberia might have Ebola. While it's still not clear what is causing his illness, tests have ruled out any of the hemorrhagic fevers, according to the Public Health Agency of Canada.

But Ebola doesn't spread very efficiently between people, Chandran said, and "everything we know about this virus suggests it's not going to spread very quickly through the human population."