The man beat cancer years ago. Why was there a mass in his lung?

Could the disease be back?

The doctor's voice on the phone was warm and reassuring. The patient, a 62-year-old man, underwent a chest CT scan earlier today and was told by Dr David Smith, his longtime PCP, that the radiologist had seen a mass. It sounded bad, but it probably wasn't cancer, Smith told her. "I didn't want you to see the report and worry," he added. The report said the mass in his lungs looked like a neoplasm – the fancy word for tumour. But he went on to acknowledge that it could also be left over from the very severe pneumonia the man had had three months earlier.

And he had been bad pneumonia. He started coughing first. Then he struggled to breathe deeply. He was burning with fever and had a shooting pain in the upper right part of his back with every breath. He tried to calm down with cough syrup and ibuprofen, but when it didn't improve, his fiancée insisted he call his doctor. The nurse who called him sent him straight to Yale New Haven Hospital. A chest X-ray showed a large white and gray cloud taking up most of the upper part of his right lung - pneumonia.

He was given antibiotics to treat a suspected bacterial infection and the next day he started to feel a little better. He was sent home to complete the five-day course. The fever disappeared, then the back pain, but the cough persisted. The simple act of breathing or talking could trigger a long period of hacking so violent that it left him breathless.

The chest x-rays repeated after one month, then two , seemed to be getting better. The cloud shrunk to a drop the size of a peanut. But while still there after three months, Smith ordered a chest CT scan. This was the report Smith was calling his patient about. The patient listened quietly but was still worried. He had had cancer once, decades earlier, and the possibility that he might have it again frightened him.

Smith knew well her patient and had previously contacted one of Yale New Haven Hospital's lung cancer experts to consult her on the need for a biopsy. She agreed with the radiologist: it was probably just leftover pneumonia. Give it a few more months, she advised, and if the lump was still there, that's when you'd do a biopsy.

When his doctor called him after the second scan and he said the lump in his chest had grown, the man felt a pang of real fear. The biopsy was uncomfortable but not painful. He lay down on his back and a long needle was inserted between two ribs. Because of the medicine he was given, he only felt intense pressure. The results were a relief. It wasn't cancer, they said. Instead, it looked like some sort of infection. A few of the samples showed odd-looking cell organisms that no one seemed to be able to identify. The pathologist sent photos of the tissue and unrecognized organisms to the Centers for Disease Control and Prevention seeking a diagnosis.

Days later, they returned their response. It was, they believed, a fungus called blastomyces. Had the patient recently traveled to the Ohio or Mississippi valleys? Or anywhere in the Midwest or the South? Blasto, as he is colloquially called, lives in the dirt there and a few other places. If inhaled, it can cause a serious lung infection called blastomycosis, which can be fatal if left untreated. Smith immediately referred the patient to the infectious disease team. The doctor on duty that week was Dr. Marwan Mikheal Azar, who luckily was an expert in fungal diseases.

Azar had just completed his training specialized. He had taken additional training in microbiology and reviewed the images that had been sent to the C.D.C. strongly. After the first look, however, he wasn't sure if the C.D.C. was right. The fungi observed on the slides were too large to be blastomyces. They were tiny organisms - less than a tenth the diameter of a human hair. The organism shown in these images was large in comparison - perhaps...