In 2011, the Berlin Zoological Garden suffered a tragic loss when internationally adored polar bear Knut had an unexpected seizure and drowned. An autopsy found that swelling in Knut’s brain, known as encephalitis, caused the seizure. But the exact cause of the swelling has remained mysterious. Today in Scientific Reports, researchers announce the culprit: Knut suffered from anti-NMDA receptor encephalitis, an autoimmune disease thought to exist only in humans. In autoimmune disorders, the individual’s immune system falsely produces defensive agents, called antibodies, which attack its own tissues. In the case of anti-NMDA receptor encephalitis, the antibodies attack a particular protein (the NMDA receptors) located on cells in the brain and the misguided attacks lead to swelling, and for Knut, an unforeseen seizure. When a nearby neurologist in Berlin read Knut’s autopsy report, he found striking symptomatic parallels to his own patients who suffered from anti-NMDA receptor encephalitis. He teamed up with those who originally investigated Knut’s death, and after running a suite of tests on stored tissue samples from the bear’s brain, the team pegged the autoimmune diagnosis. The discovery suggests that anti-NMDA receptor encephalitis is far more common in mammals than previously thought. Scientists hope that because the disease is treatable in humans, they’ll be able to diagnose and treat animals in captivity before it kills them.