A mysterious ailment is turning some black bears friendly and unafraid of humans, among other strange symptoms.

Researchers believe the illness, which appears to afflict young bears around a year old, is a kind of encephalitis, meaning an inflammation of the brain, according to a news release from the California Department of Fish and Wildlife late last month. Since 2014, a handful of bears in Nevada and California have shown signs of the condition, and four California bears were confirmed to have it in the last year.

The first bear I personally rescued was diagnosed with encephalitis in March of 2018, Ann Bryant of the Bear Education Aversion Response told Live Science . She walked into a school and entered a classroom where she sat down among the children, behaving very much like a friendly dog.