Steven Francis has been sick since as long as he can remember.

Steven Francis, pictured here as a child with Expos mascot Youppi, spent most of his childhood at the Montreal Children's Hospital. (submitted by Steven Francis) It started when he was just six months old, with chronic vomiting, diarrhea and fevers.

He spent most of his childhood at the Montreal Children's Hospital, where he faced sinus infections, fungal infections, shingles, respiratory problems, kidney problems and impeded growth.

Doctors did their best, but no one could figure out exactly what was wrong with Francis.

"It was pretty frustrating, not knowing what I had. What's the illness? What's causing it? Why am I getting sick so often? Why am I not the same as other people?" Francis told CBC.

"I was always a bit of a mystery. I was like a mystery boy."

Enter Dr. Vinh

Francis figured he would never know what was wrong with him, and he did his best to cope with his parade of illnesses.

In 2012, he was referred to Dr. Donald Vinh, a specialist in infectious diseases and genetic defects at the McGill University Health Centre.

Since he was a student, Vinh has been interested in medical cold cases: illnesses that other doctors just can't seem to solve.

"I always had the burning curiousity of why were certain patients getting certain infections, and other people who were exposed to the same organisms, they weren't getting those infections?" Vinh said.

Patients with these mystery diseases are like orphans in the health care system, he said.

"When people are chronically sick, or when they have a genetic problem with their immune system that has gone unrecognized for years or even decades, it can be infuriating," Vinh said.

"They also feel very isolated."

After 35 years, medical cold case solved

Francis was referred to Vinh to treat a blood infection.

But Vinh's curiosity took hold. He reviewed Francis's entire 35-year medical file, studied his family history, did extensive blood work and conducted detailed genetic tests.

Dr. Vinh told CBC life for patients with mystery illnesses, such as Steven Francis, seen here as a boy, can be infuriating - and lonely. (Steven Francis) It took some time, but last week Vinh was finally able to tell Francis what had been making him so sick.

"His gene was faulty, and it wasn't producing enough protein for one of his cells to be able to function properly," Vinh said.

"It was a very subtle mutation. We can understand why it had been missed for thirty years, but we found it, and we were able to prove that it was faulty."

Now that Vinh has identified the genetic mutation, he believes he may already have a fix: a molecule developed in the lab that can correct this kind of genetic anomaly.

It's not ready for use on patients yet, but Vinh believes within a few years such a treatment may be able to help Francis.

'This is what we live for'

"It's overwhelming. It's pretty amazing to find out what I have, after 35 years," Francis said. "Hopefully things will turn around now for the best."

Vinh said his work on Francis's case will likely lead to treatments for other patients with complicated health problems linked to their immune systems.

"If people fall into these categories: recurrent infections, problems with their immune system, problems with fevers, histories of cancers, those are the type of people I think we can help, particularly," Vinh said.

For Vinh, helping patients such as Francis means everything.

"It's my raison d'être. This is what we live for," he said.