There is no overstating the fear that the Ebola virus inspires when it can kill up to 90 percent of those infected.
But epidemiologists say that there is little likelihood that Ebola -- which has killed 729 people in four West African countries since it was detected in February -- will spread across the globe in the kind of pandemic seen in 2009 with the swine flu virus.
Fears that Ebola might leapfrog across continents rose after a man infected with Ebola died July 25 after flying from Liberia to Nigeria via Togo. His case underlined the difficulty of screening passengers traveling out of the outbreak area and potentially carrying the virus with them, much the way air travelers spread the H1N1, or swine flu virus, from Asia in 2009.
However, experts say Ebola is unlikely to spread as easily as swine flu because it is much more difficult to transmit between humans.
"Ebola and infections like Ebola are spread mostly by direct contact with body fluids, whereas swine flu and SARS and similar influenza-like illnesses are spread through the air," says Nick Beeching, a senior lecturer and consulting physician at the Liverpool School of Tropical Medicine in England.
Instead, Ebola may be better compared to the HIV-AIDS virus, which also passes through direct contact with body fluids. But here, too, there is a difference that suggests Ebola poses less danger of an epidemic.
"HIV is infecting millions across the Earth right now but Ebola has infected fewer than 5,000 people that we have known about in all of recorded history, even with this latest outbreak, which is the largest one," says Ben Neuman, a virologist at the University of Reading in England. "Ebola is more fatal on a case-by-case basis but it does not spread very well and it has always been contained in sub-Saharan Africa."
Neuman says that one reason Ebola does not spread quickly is its fearsome mortality rate. On average it kills 60 percent of its victims, with the incubation period lasting from just two to 21 days.
"With Ebola you have a virus that essentially either kills the host fairly quickly or the host survives and conquers the virus," Neuman says. "Either way the virus has come to a dead end if it hasn't spread to more people and this virus is not very transmissible."
Epidemiologists believe the Ebola virus evolved within various animal populations living in the tropical rainforest of equatorial Africa, notably fruit bats. It is believed to pass to humans either through contact with the bodies of dead animals that carry the virus or possibly through direct consumption of the animals as food.
Outbreaks of the virus, which was first identified in 1976 in Zaire, today the Democratic Republic of the Congo, usually take place in remote rural areas where health care resources and personal hygiene practices are minimal. Since its discovery, there have been periodic epidemics in the Congo and neighboring Uganda and South Sudan but never before in West Africa.
Experts say the fact the outbreaks occur in remote rural areas gives the virus a chance to spread to levels it would not be able to achieve in other environments, where better health practices, including quarantining infected persons, would help stop it.
"In countries like Guinea and Liberia and Sierra Leone their health services are very stretched and also there is understandably a lot of fear in the local population who are not always very keen on being quarantined," says Beeching.
Distrust of health workers may also help to spread the disease. Villagers in some of the outbreak region in West Africa have refused to give health workers access to their areas because they believe the doctors themselves are carrying the disease from one place to another.
The recent outbreak in West Africa has not only killed far more people than the previous record of 435 deaths in Uganda in 2000, it also marks the first time Ebola has moved from rural areas to large urban centers.
On July 30, Sierra Leone declared a public health emergency and announced it would quarantine the epicenters of the outbreak in the east of the country, with security forces deployed to enforce the measures.
There is no vaccine or cure for Ebola, whose symptoms include fever, sore throat, and headaches, and can initially be confused with those of a bad cold. As the infection progresses, patients suffer vomiting, diarrhea, and liver malfunction and, in some cases, internal and external bleeding.
The chances of survival can be improved by ensuring a patient is fully hydrated, since loss of fluids is often the direct cause of death.