Reported by Susan Kreimer, ABC News Medical Unit:

The death toll from the Ebola virus has doubled within a week in the Democratic Republic of Congo, and the outbreak may continue spreading, the World Health Organization cautioned on Thursday.

The Ministry of Health has confirmed 18 deaths among 41 cases of the highly contagious virus.

"Up to 90 percent of those who are infected may die," said Tarik Jasarevic, a spokesman for the World Health Organization in Geneva, Switzerland. "That's why every outbreak is serious."

Outbreaks tend to be localized because victims of the virus are generally too ill to travel, so the risk of exposure in the United States is extremely unlikely, Jasarevic said.

Reaching epidemic levels in northeastern Congo, the Ebola virus has stricken people in the towns of Isiro and Viadana in Orientale province, which borders southern Sudan and northern Uganda.

Its incubation - the time between infection and appearance of symptoms - lasts between two and 21 days. Death can occur within a few days.

"The incubation period can be long," Jasarevic said. "But once you start showing symptoms, it goes very fast."

Onset of illness may manifest in the form of fever, headache, joint and muscle aches, sore throat and weakness. Other early symptoms, such as red eyes and a skin rash, are nonspecific to the virus and can be present in diseases that occur much more commonly, according to the U.S. Centers for Disease Control and Prevention.

The illness progresses quickly to diarrhea, vomiting, stomach pain, impairment of kidney and liver function, and internal and external bleeding, which is why it is also called "hemorrhagic fever."

Ebola outbreaks have been documented since 1976. In addition to the Congo, they have struck in Uganda this year. In other years, outbreaks have occurred in Sudan, Gabon and the Ivory Coast.

"In Uganda, the outbreak is coming to an end," Jasarevic said.

The Ebola virus takes its name from a river in the Congo (formerly Zaire), where it was first identified.

In collaborating with the World Health Organization, the CDC has developed practical, hospital-based guidelines for infection control in the African health care setting.

The manual recommends using common and low-cost supplies, such as household bleach, water, cotton cloth and plastic sheeting to curb infection. It is available in English, French and Portuguese.

Anyone - from the very young to the very old - can be infected with Ebola.

The virus is first transmitted from animals to humans, typically by hunters who kill primates in the jungle and then consume infected meat, Jasarevic said. Carriers of the virus can infect others through bodily fluids, including saliva, sweat, nasal drip and blood.

There is no standard treatment and no vaccine. Patients receive supportive therapy to balance their fluids and electrolytes, control blood pressure and oxygen levels, and manage any infections that may occur, according to the CDC.

Researchers do not know why a small percentage of people recover from Ebola while others do not. Those who die usually have not developed a significant immune response to the virus.