“A lot of mom-and-pop patient organizations emerge to take on these huge challenges in rare diseases,” he said. “I don’t think there’s one correct model for each disease; there’s been so many different approaches.”

Last month, Dr. Fajgenbaum marked his three-year anniversary since starting on sirolimus, a period more than twice as long as any of his other remissions. “I feel 100 percent,” he said.

These days, Dr. Fajgenbaum, now 31, walks through the hallways of Penn’s medical center, his frame again projecting the easy confidence of the athlete he once was. But he jokes that he would have to pull out photos of his days as a quarterback to explain to people why his friends still call him the Beast.

Now, he said, every time he tries to exercise, his mind wanders back to an email he needs to write or a call he needs to make. “It’s not because I don’t have the energy to do it; it’s because all of my energy is going toward this disease,” he said.

Not everyone is convinced that sirolimus is what has been keeping him healthy. Dr. van Rhee noted that while the results in Dr. Fajgenbaum are promising, his is just one case and treatments need to be proven in many more people. “I think the finding is very interesting,” he said, “but we need to see whether a similar mechanism is active in other patients.”

But Dr. Fajgenbaum said he grew more confident every day that it is the drug that is helping. He has begun sharing his experience with more doctors and researchers, is conducting laboratory tests to see if the drug is likely to work in other patients and has started writing an article about his experience for a medical journal. Soon, he hopes, doctors will begin prescribing the drug to other patients.

Dr. Fajgenbaum is optimistic about the drug’s chances but is aware medical science is unpredictable. “Who knows; maybe it will work for only a small percentage,” he said. “So we’ve still got a lot of work ahead of us.”