Scientists suspect that a new milder strain of the Ebola (search) virus may have caused the latest outbreak of the deadly hemorrhagic fever in southern Sudan (search), the World Health Organization said Saturday.

Four of the 10 people infected with the Ebola-like virus have died in Yambio, a Sudanese town near the border with Congo (search), said Abdullahi Ahmed, head of WHO's southern Sudan office.

Tests of blood samples taken to the U.S. Centers for Disease Control in Atlanta indicated that the oubreak is not linked to the known strains of Ebola-like viruses that cause the severe viral infection, Ahmed said.

There are four known strains of Ebola-like viruses, three of which cause the deadly disease, the CDC said on its web site. The viruses are probably preserved in an undefined reservoir in the rain forests of Africa.

"The investigation could not match the known variants, so it could be a new variant," Ahmed told The Associated Press.

Scientists at the CDC will conduct genetic tests to characterize and classify the suspected new strain of the Ebola virus, Ahmed said.

So far the death rate is lower than has previously been experienced with Ebola outbreaks, he said, adding that this may indicate that the new virus is milder than other strains.

The suspected new variant has killed between 25 and 30 percent of the people infected, while other variants kill between 50 and 90 percent of those who contract the disease, Ahmed said.

The symptoms of the illness around Yambio include general malaise, fever, vomiting blood and bloody diarrhea.

There had been no new cases reported in the last four days, Ahmed said, adding that one person remains under investigation. The incubation period for the virus is 21 days. The most recent case began on May 15.

Southern Sudan has been wracked by civil war since 1983; thousands of people are periodically displaced by fighting, and public health facilities are rare.

The first recognized Ebola epidemic occurred almost simultaneously in southern Sudan and in a nearby region of the Congo in 1976, according to the CDC.

At the time, Ebola killed 117 of the 284 people who were reported with the viral infection in southern Sudan. In Congo, it killed 280 of the 318 people infected.

In 2000, an Ebola outbreak killed 173 people in Gulu district in northern Uganda, which also borders southern Sudan.

The Ebola virus is spread by contact with body fluids, including sweat and saliva. Outbreaks of the disease are rare, and no one knows where the virus lives when it is not infecting humans. The disease usually kills its victims so fast that it also destroys the host for the virus.

Ugandan health officials said they had circulated warnings in areas that border Sudan and had put health workers on high alert.