Doctors thought he just had jock itch. Then it spread.

Late Friday afternoon on Dec. 4, 2014, Stephen Schroeder was waiting to board his packed flight from Philadelphia to Las Vegas for a much anticipated long weekend with his son when his cellphone rang. On the line was an unexpected caller: his doctor, reporting test results sooner than Schroeder had expected.

Listening intently, Schroeder was flooded with disbelief as he struggled to comprehend what he was hearing. Using the lip of a trash can as a writing surface, he scribbled notes on the back of his boarding pass, making the doctor spell out each unfamiliar word. Then he sent a terse text to his wife, who was at their home in the Philadelphia suburbs, and got on the plane.

Onboard, Schroeder, then 55, fired up the balky in-flight Internet, desperate for information.

What he read over the next five hours left him alternately terrified, stunned and then, as denial took over, skeptical. “I kept thinking this must be some kind of really stupid mistake,” he recalled. “The diagnosis had to be wrong.”

Schroeder would discover that the pesky rash he and his doctors had dismissed as inconsequential would take over — and threaten — his life.

The experience would provide a crash course in the importance of finding experts who could provide appropriate treatment, in the necessity of learning as much as possible about a disease, and in the loneliness of coping with a diagnosis so rare it lacks a support group. A case of jock itch

In the fall of 2013, Schroeder noticed a small, purplish pimple on his groin. “I was pretty sure it was an ingrown hair, so I ignored it for six months,” figuring it would go away on its own, he recalled. The pimple disappeared, replaced by a dime-size red, scaly rash on his scrotum. Schroeder said he assumed it was jock itch — vernacular for a common fungal infection of the groin — and did nothing for several more months.

“I’m a guy,” he said. “It didn’t faze me much.” The director of membership development for a purchasing cooperative, Schroeder had been successfully treated for melanoma, the deadliest form of skin cancer, in 1989. He had not had a recurrence and was always vigilant about annual checkups.

In the spring of 2014, as his primary-care doctor was on her way out the door after a routine appointment, Schroeder said he remembered the red patch “almost as an afterthought” and mentioned it.

She examined the spot, agreed it looked like jock itch and suggested he try the standard treatment: an over-the-counter antifungal cream. Several weeks later, when the problem remained, Schroeder returned. His physician and one of her partners took a look and prescribed a stronger antifungal medicine.

“They both agreed it was nothing to worry about,” Schroeder recalled. “It wasn’t painful, just annoying” and itchy.

The second medicine was no more effective than the first. Schroeder then consulted his dermatologist. He, too, initially agreed it was a stubborn fungal infection, which thrives in moist, dark areas of the body. Later, the dermatologist changed his mind and suspected eczema, a skin inflammation that causes redness and itching. Another possibility, he told Schroeder, was that he had developed contact dermatitis, a common allergic reaction to the shampoo, soap or fabric softener he used.

Schroeder thought that was odd, but he changed his shampoo and soap brands and stopped using fabric softener. “I think he was winging it as he went,” Schroeder said of his dermatologist. The rash didn’t improve.

Fifteen months after the first symptom and a few days before his Las Vegas weekend, Schroeder returned to the dermatologist and asked him to perform a biopsy to determine the cause of the red patch.

“Steve, this is kind of weird,” he remembers the dermatologist saying when he answered his phone at the airport.