A healthy young man got a simple bug bite that turned life-threatening after it developed into a flesh-eating bacterial infection

A bug bite is usually an itchy annoyance for a few days until it goes away. But for a 21-year-old man, a simple bite on his knee turned into a flesh-eating bacterial infection.

The young man first went to the emergency room because his right knee was swollen and painful, making it difficult for him to walk. According to his doctors, who wrote about the incident in a new case study published in the American Journal of Emergency Medicine, the man said he hadn’t hit or hurt his knee in any way, but he had gotten a bug bite three days before.

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“I was really surprised to see that this otherwise young and healthy guy could barely walk,” Dr. Jacqueline Paulis, an emergency medicine physician at New York University School of Medicine who treated the patient, and is the lead author of the case study, told LiveScience.

The fact that he couldn’t move his knee “raises a lot of red flags for doctors that there’s something deeper going on,” Paulis said.

Doctors performed an exam and found that the bite was giving off pus, plus the tissue around his knee was starting to die off. The man’s health problems were also spreading through his body. He was feeling sharp pains in his chest every time he drew in a breath, and had flu-like symptoms.

The x-ray also showed air underneath the dying tissue, an indicator of flesh-eating bacteria, or necrotizing fasciitis. Necrosis is the death of living cells or tissue, and is extremely aggressive — and can be deadly — if left untreated.

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Along with the flesh-eating bacterial infection, the man also had a septic pulmonary embolism, another rare disease where the infection moves to the lungs and causes a blockage and blood clotting.

In this man’s case, the flesh-eating bacteria likely was not from the bug itself, but from scratching the resulting bite, which tears at the skin and exposes it to infection.

What was odd about this case is that healthy bodies like the 21-year-old’s can typically contain the bacteria and defeat it.

“Usually our bodies and immune systems are healthy enough to contain that and mitigate it,” Paulis said. “He was the epitome of health, 21 and young.”

But thanks to the early detection, doctors were able to treat the man with antibiotics and remove the dead tissue in his knee. After a few weeks in the hospital, he fully recovered and was able to go home.

Paulis said she wanted to share this case study as a warning to her fellow doctors that flesh-eating bacteria can combine with a pulmonary embolism.