It’s estimated that somewhere between one and five million people are bitten by snakes every year, with around 1/5 of those resulting in death. That number is a lot lower than it once was — several decades ago, antivenoms for deadly snakes were few and far between, so people frequently succumbed to bites. One such victim was American herpetologist Karl P. Schmidt.

Schmidt worked for the American Museum of Natural History in New York and the Field Museum in Chicago during his scientific career, and even was president of the American Society of Ichthyologists and Herpetologists from 1942 to 1946. He had handled countless deadly snakes. But in 1957, he made the mistake of underestimating a juvenile boomslang that Marlin Perkins, then the director of the Lincoln Park Zoo, had sent him for identification. He didn’t believe the snake could inject a lethal dose, so he didn’t seek medical treatment until it was too late. Ever the scientist, Schmidt documented the last 15 hours of his life in his diary, which Science Friday has made into a harrowing video.

Warning: video contains graphic descriptions.

Boomslangs are one of few deadly members of the snake family Colubridae, often referred to as the rear-fanged snakes for their uniquely-positioned dentition. Unlike the other two major venomous groups — the Viperidae, or vipers, and the Elapidae, or elapids — the colubrids’ fangs are located at the rear of the jaw, and in most species, the fangs are too small and their venom too weak to cause much damage in people. But the boomslang defies colubrid convention: it can open its mouth to almost a 180° angle when biting, thus allowing it to sink its fangs deep into our flesh, and is armed with a blood-curdling venom more deadly than the venoms of cobras or even the notorious black mamba when injected into mouse veins.

If Schmidt had rushed to the hospital, it’s possible that medical intervention could have saved his life — so his pride and belief that the small snake simply couldn’t kill him may have contributed to his death. But without boomslang antivenom — which, at the time, was only available in Africa — doctors would have been fighting an uphill battle from the get go. Nowadays, zoos and other facilities which keep venomous snakes always have a stock of antivenom at the ready, just in case.