FREETOWN, Sierra Leone — Joseph Fair hunts viruses. That’s his thing. The 37-year-old American loves chasing dangerous pathogens, studying them in secure labs or searching for them in jungles where the microbes lurk.

And one virus has always loomed as the big one — Ebola. The scientists who first chased this dreaded microbe back in the ’80s and ’90s became legends, inspiring a generation of virologists like Fair. He read their books and papers. He studied how they contained the pathogen’s spread. And the scientists always won. The outbreaks ended, Ebola driven away.

So when the call came in March to travel to Sierra Leone, Fair was excited. He loved Mama Salone, as locals know the nation. He’d worked here for years. His new job: to advise Sierra Leone’s government on a tiny Ebola outbreak in neighboring Guinea, at the behest of the U.S. Defense Department. He set up an Ebola emergency operations center. He trained medical staff. He drew up just-in-case plans. By mid-May, the outbreak seemed on its way out. Fair packed his bags and left.

Then Ebola exploded.

Now, Fair is back in West Africa, in the middle of the worst Ebola outbreak in history. A viral epidemic. The world’s ability to respond is stretched. Plans are being devised on the fly.

“This is the big one no one expected,” Fair says.

It has been a humbling time. Fair has worked seven days a week, hunting for ways to curb the outbreak, at times begging international groups for staff and supplies. He is exhausted. He’s put on 15 pounds. He sent his girlfriend home to California months ago. Too dangerous here, he told her. He’s seen Sierra Leonean doctors and nurses — friends he’s known for years — get infected and die. He sweated out his own Ebola scare.

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