The international fight against the deadliest Ebola outbreak in history was dealt another major blow Tuesday with the death of Sierra Leone’s top Ebola doctor, Sheik Umar Khan.

Khan, 39, was working with the Doctors Without Borders, or MSF, team in his country fighting the massive outbreak that’s killed more than 200 Sierra Leoneans and over 660 people total across West Africa. Sierra Leone's chief medical officer confirmed that Khan succumbed to the Ebola virus on Tuesday.

Khan is credited with treating more than 100 Ebola patients. Health Minister Miatta Kargbo called him a “national hero.”

Khan was known for being extremely meticulous about his protective clothing and taking all precautionary measures to make sure he didn’t catch the virus. Prior to falling ill, Khan expressed his fears of catching Ebola, even with all of his precautions.

“I am afraid for my life, I must say, because I cherish my life,” Khan said. “Health workers are prone to the disease because we are the first port of call for somebody who is sickened by disease. Even with the full protective clothing you put on, you are at risk.”

Ebola symptoms occur soon after the virus is contracted, and the infection kills extremely quickly. Within two days to two weeks of contracting the virus, victims experience extremely high fevers, vomiting, diarrhea, as well as external and internal hemorrhaging. Some outbreaks have had a 90 percent mortality rate, but the current outbreak’s mortality rate sits around 60 percent.

One of Liberia's top Ebola doctors, Dr. Samuel Brisbane, was also killed by the virus over the weekend. Two American aid workers were also infected; Dr. Kent Brantly is in "grave condition" while nurse Nancy Writebol is in serious condition.