The third American aid worker who contracted Ebola in Africa was released Thursday from a Nebraska hospital.

An elated Dr. Rick Sacra said at a news conference that he was cleared of the Ebola virus by the federal Centers from Disease Control and left the isolation unit at the Nebraska Medical Center on Thursday morning.

"I feel great, except that I am extremely weak," Sacra, a 51-year-old doctor from Worcester, Massachusetts, said at a press conference. He added he has worked his way up to riding five minutes on an exercise bicycle before becoming too tired.

Sacra said he's been in communication with fellow Ebola survivor Dr. Kent Brantly, and based off their conversations he is expecting a long recovery ahead.

Sacra, who was working at a hospital in Liberia with the North Carolina-based charity SIM, contracted Ebola while treating sick pregnant women.

His symptoms first began with a fever that lasted for three days before exhibiting other Ebola symptoms. Sacra had quarantined himself when he first felt ill.

"When I continued to have a fever on the third day, they decided to run my Ebola test," he said. Sacra was then flown to Nebraska on Sept. 5, where he immediately began treatment and showing signs of improvement in a special isolation unit.

"I was here on day six. I was receiving medications, I was receiving plasma from my friend Dr. Brantly," Sacra said of his treatment..

"I never felt like I was not going to make it. The care was so excellent, so speedy and so prompt," Sacra said.

The World Health Organization says the Ebola virus is believed to have killed more than 2,900 people in West Africa. Governments are scrambling to contain the disease outbreak, and the United States has promised to send 3,000 soldiers to the region to help.

"Unfortunately what we discovered is this epidemic is so widespread now that even people who don't have the classic symptoms that you were looking for are sometimes the people who have Ebola," Sacra said.

Sacra said seeing more Ebola patients be treated and ultimately released is helping to encourage others in West Africa to seek medical help.

"More and more people are realizing that if they get into care they will probably have close to a 50/50 chance of survival," Sacra said, adding that while the odds are not great it is better than those not seeking treatment at all.

"Every health facility in Liberia lost multiple workers in July when the crisis ramped up," Sacra said. "When you have seen your colleagues get ill and die from Ebola, it makes it hard for you to go back to work," he said, adding that the medical worker shortage highlights the urgency of a need for a vaccine.

"Though my crisis has reached a successful end here, unfortunately the Ebola crisis continues to burn out of control in West Africa," Sacra said. Despite his ordeal, Sacra will not rule out a return to Liberia, which he called a "second home."

Two other American doctors who contracted Ebola -- including Brantly -- were treated at Emory University Hospital in Atlanta, and were released after recovering. A fourth American with Ebola is still being treated in Atlanta.

Dr. Phil Smith has said Sacra received an experimental Tekmira Pharmaceuticals drug called TKM-Ebola for a week after he arrived in Omaha. Sacra also received two blood transfusions from Brantly. These blood transfusions are believed to help a patient fight off the Ebola virus because the survivor's blood carries antibodies for the disease.

Sacra also received supportive care including IV fluids and aggressive electrolyte management, and his own immune system fought the virus.

Doctors have said that the combination of treatments Sacra received makes it difficult to know what helped him fight off Ebola.

The Associated Press contributed to this report.