More cases of Ebola have been reported in West Africa, but this time it's two U. S. Peace Corps volunteers who may be affected. This is the largest outbreak of Ebola ever recorded, according to the World Health Organization.

The volunteers have not shown signs of the disease, but have been isolated for exposure. The incubation period can be up to three weeks for Ebola.

They are said to have come into "contact with an individual who died of the Ebola virus," said a Peace Corps spokeswoman to CBS news.

"The agency has been and will continue to closely monitor the outbreak of the virus in collaboration with leading experts from the (U.S.) Centers for Disease Control and Prevention and the U. S. Department of State," the Peace Corps said.

The Peace Corps is now temporarily removing 340 volunteers from Liberia, Sierra Leone and Guinea in order to prevent any more volunteers contracting the disease.

This comes after two other Americans, aid workers Dr. Kent Brantly and Nancy Writebol, contracted the disease and remain in serious condition, although they have shown some improvement within the last 24 hours. One American has died from the disease.

"We ask that people continue to pray for Kent and Nancy and all those who are affected by Ebola, and the tremendous group of doctors and nurses who are caring for them," Samaritan's Purse said in a statement. Brantly is a doctor with the Christian aid group, while Writebol is a missionary.

The disease primarily spreads through exposure to bodily fluids even after death. So far, more than 672 people have died in this outbreak across Guinea, Liberia and Sierra Leone.

The risk of travelers contracting Ebola is considered low because it requires direct contact with bodily fluids or secretions such as urine, blood, sweat or saliva, experts say. Ebola can't be spread like flu through casual contact or breathing in the same air.

Patients are contagious only once the disease has progressed to the point they show symptoms, according to a WHO fact sheet. And the most vulnerable are health care workers and relatives who come in much closer contact with the sick.

Brantly is currently being treated at ELWA Hospital, Samaritan Purse's hospital in Liberia. His family flew back to the United States before he began showing symptoms of the disease, but they are on fever watch in Texas where they are visiting for the next three weeks as an extra precaution.

Writebol is in isolation, but she is reported able to move around the home where she is being treated and has been getting plenty of fluids.

Patrick Sawyer, the American who died from the disease, flew from Liberia to Nigeria, where he died in quarantine. He had been a consultant in Liberia where he had visited his sister, who died from the disease, according to reports.

The CDC has issued an Alert Level 2, Practice Enhanced Precautions, Travel Health Notice for the areas affected with Ebola on Monday, July 28.

Currently, most of the borders into and out of Liberia have been closed off with very few points of entry. The country has closed its school and is advising residents to avoid public amusement and entertainment areas.

This outbreak of the Ebola virus has had a fatality rate of 60 percent, but is known to have up to a 90 percent mortality rate. There is no vaccine. Those infected can recover, but must stay hydrated and rest.

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