Looking at me now, people would never suspect I’d ever had anything serious wrong with me. I walk five miles every day and take the stairs up to my third-floor apartment with ease.

But in April 2018, I was so weak I could barely get out of bed. And I couldn’t take more than a few steps without sitting down to rest. At the time, I was undergoing nine weeks of very strong chemotherapy for stage IV nasopharyngeal carcinoma, a rare...