A 70-year-old patient who presented with cancer-like symptoms turns out to have been harbouring something even weirder: a fungal infection that may have been in his body for 30 years before making itself known.

It turns out what he had was a disease called histoplasmosis - a condition caused by inhalation of the spores of a fungus called Histoplasma capsulatum.

He presented to the hospital complaining of "altered mental status" for four days, according to the case report - he seemed otherwise normal, but a bit confused. A CT scan, followed by an MRI, showed a lesion on his brain, and his doctors feared the worst: metastatic malignancy.

When they conducted further MRI scans, they also found masses in both his adrenal glands. But they did not suspect histoplasmosis. Partially because histoplasmosis, as it is inhaled, mainly presents in the lungs.

Mainly, however, because the patient lived in Arizona, and had not left the state for many years. Histoplasmosis, also known as cave disease, is fairly common in areas such as the Mississippi and Ohio river valleys, but not Arizona.

The humidity of the locations where it is found, the acidity of the soil, and the presence of bird and bat droppings all contribute to the fungus thriving. Not everyone who inhales the spores contracts histoplasmosis, either.

In fact, the incidence of histoplasmosis is relatively low, occurring in 3.4 cases per 100,000 in people 65 and over, the researchers said.

By far the worst affected by the disease are people who are immunocompromised. In places like the Mississippi and Ohio river valleys, 5 percent of people with AIDS contract disseminated histoplasmosis - when it spreads from the lungs to other parts of the body.

When the doctors conducted a biopsy in the adrenal masses of their 70-year-old patient, they found granulomas consistent with disseminated histoplasmosis.

Why the man was susceptible to it was fairly clear: in 1986, he'd had a heart transplant, and was on long-term immunosuppressants so that his body would not reject the transplanted organ as foreign tissue.

But how the spores got into his body in the first place remained a mystery. Usually symptoms of the disease appear between 3 and 17 days of inhaling the spores.

Yet the patient reported that he had not been near bird or bat droppings, and had not been to an area where the fungus is prevalent.

The most recent visit he had made to a location where he might have picked it up - at least that he could recall - was a brief visit to North Carolina, 30 years ago.

The patient suffered complications from the biopsy of the brain lesion, and later tests also revealed more masses in his liver. Eventually, he went home on palliative care and continued to take an antifungal medication.

As the doctors wrote in their report, this is a unique case because the patient had no previously diagnosed infection and remained symptom-free for so many years.

The case, the doctors wrote in their report, was a reminder to leave no stone unturned, citing a 2016 report on histoplasmosis in areas where the disease isn't prevalent.

The researchers found that in 11 out of the 20 cases of histoplasmosis they analysed, the patient did not report travelling to an area where the fungus is common.

The case report has been published in the journal BMJ Case Reports.