Intensive testing: There was no doubt that Mr. Stanacev had meningitis — his brain was inflamed. But why? Antibiotics did not help, and neither did steroids, which should tamp down an inflammation.

No matter how many times he was tested, doctors found no sign of a bacterial or viral infection. Finally, late last year, Mr. Stanacev was referred to the Undiagnosed Diseases Network’s site at the N.I.H.

He received the full gamut of testing: imaging, blood draws, genetic analysis and, importantly, a lumbar puncture to obtain cerebrospinal fluid, which bathes the brain. That fluid showed clear signs of extensive inflammation.

At the end of the week, the Stanavecs went home, praying for a diagnosis.

The diagnosis: The Stanavecs got a phone call in April from the clinic. Researchers had figured out what was wrong.

Mr. Stanavec had an extremely rare mutation in a gene, NLRP3, which helps direct cells to activate a protein, interleukin 1 beta, that is part of the immune response to infections. The mutation made him produce an NLRP3 protein that was always active — even when there was no infection.

There are just two or three reported cases of this mutation in the medical literature, said Dr. William Gahl, clinical director of the National Human Genome Research Institute at the N.I.H. It’s not clear why Mr. Stanavec’s symptoms started so late in life, Dr. Gahl said. Nor is it clear why only his brain was inflamed.

But the good news for him was that there is a drug on the market — anakinra, used to treat rheumatoid arthritis — that blocks interleukin 1.