(CNN) -- Humanitarian agencies working in west Africa on Wednesday stepped up their precautions against the Ebola virus, with the Peace Corps announcing it was removing hundreds of its volunteers from three of the hardest-hit nations.
The Peace Corps said 340 volunteers would temporarily leave Liberia, Sierra Leone and Guinea, as local, national and international organizations tried to contain the disease.
"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.
Two American aid workers working with Samaritan's Purse who were infected with the deadly Ebola virus "have shown a slight improvement in the past 24 hours," according to that international evangelical Christian humanitarian agency.
Both remain in serious condition, the agency said.
Dr. Kent Brantly, 33, who last lived in Fort Worth, and Nancy Writebol, who is from Charlotte, North Carolina, sought treatment for exposure to the virus last week. Both were caring for patients with Ebola in Monrovia, Liberia's capital.
"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.
Doctors and other medical staff are particularly vulnerable to the Ebola because it spreads through exposure to bodily fluids from the infected. It can also spread through contact with an object that has been contaminated by an infected person's bodily fluids.
Brantly works with Samaritan's Purse. He has been the medical director for the Ebola Consolidated Case Management Center in Monrovia, where he has been providing care for Ebola patients since October. After testing positive for the virus, Brantly went into treatment at ELWA Hospital.
Samaritan's Purse has been working to evacuate him for better care, said Ken Isaacs, vice president of the agency. Unfortunately, emergency medical evacuation flights in the area are not equipped to handle the "intense isolation" required for an Ebola patient.
Brantly's family had been with him in Liberia, according to the Centers for Disease Control and Prevention, but left for the United States before he became symptomatic; it is highly unlikely that his family members caught the virus from him, the CDC said. Out of an abundance of caution, they are on a 21-day fever watch, the CDC said.
"We have a strong family unit within a stronger faith community that has given us incredible support," the Brantly family said in a statement Tuesday. "Kent remains very physically weak, but his spirit has been determined throughout this ordeal."
Writebol works for Serving in Mission, or SIM. The missionary organization had teamed up with the staff from Samaritan's Purse to help fight the Ebola outbreak when she got sick.
It is believed one of the local staff was infected with Ebola and came to work with the virus on July 21 and 22, Isaacs told CNN.
"We think it was in the scrub-down area where the disease was passed to both Nancy and Kent," he said. That staff member died Thursday.
Because of the uptick in Ebola cases in the region, Todd Shearer, a spokesman for Samaritan's Purse, told CNN that the organization is evacuating nonessential staff out of Liberia. Serving in Mission also has recalled all nonessential personnel from Liberia, according to its website.
On Monday, the CDC issued an alert warning travelers to avoid hospitals with Ebola patients and funerals for those patients in Liberia, Sierra Leone and Guinea due to the outbreak. The United States is considering raising the alert to discourage "nonessential" travel to those three countries, a U.S. government spokesman said.