(CNN) -- A doctor who was on the front lines fighting the Ebola outbreak in Sierra Leone has died from complications of the disease, Doctors Without Borders said Tuesday.

Dr. Sheik Humarr Khan fell ill early last week while overseeing Ebola treatment at Kenema Government Hospital, about 185 miles east of Sierra Leone's capital city, Freetown.

He was treated by the French aid group Medecins Sans Frontieres -- also known as Doctors Without Borders -- in Kailahun, Sierra Leone, up until his death, spokesman Tim Shenk said.

"RIP Dr. Sheik Humarr Khan," wrote one of the many people who remembered Khan fondly on Twitter. "What a hero. What a loss."

A University of Sierra Leone graduate, Khan worked for that African country's Ministry of Health and Sanitation, including as head of the Lassa fever program at Kenema Government Hospital, according to the Viral Hemorrhagic Fever Consortium.

Lassa fever is a virus common in west Africa, the U.S. Centers for Disease Control and Prevention notes.

While in that latter post, Khan contracted with the United Nations Mission in Sierra Leone, consulted with the World Health Organization/Tulane University on its Mano River Union Lassa fever network and was physician-in-charge of his hospital's HIV/AIDS program.

"His valiant work will not be forgotten," Dr. Penninah Iutung Amor, AIDS Healthcare Foundation's Africa bureau chief, said in a statement. "In a country that has fewer than 200 medical doctors in its entire public health sector, the loss of even one doctor is a loss too great."

Khan continued his training in Ghana from 2010 to 2013, before returning to head the Kenema Government Hospital's Lassa fever program and becoming a lecturer at the University of Sierra Leone. The Viral Hemorrhagic Fever Consortium described him as "one of the world's leading experts in the clinical care of viral hemorrhagic fevers" -- among them, Ebola.

This disease typically kills 90% of those infected, but the death rate in this outbreak has dropped to roughly 60% because of early treatment. The outbreak is happening primarily in three West African countries: Guinea, where it began, Liberia and Sierra Leone.

As of July 23, the World Health Organization had confirmed more than 800 Ebola cases in the region, but it suspects there have been many unreported infections and there may be as many as 1,200 cases.

Sierra Leone has been hardest hit, with approximately 525 cases.

"Dr. Khan was an extremely determined and courageous doctor who cared deeply for his patients," Doctors Without Borders said in a statement.

"His work and dedication have been greatly appreciated by the medical community in Sierra Leone for many years. He will be remembered and missed by many, especially by the doctors and nurses that worked with him. MSF's sincere thoughts and condolences are with Dr. Khan's family, friends and colleagues."

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CNN's Greg Botelho contributed to this report.