The oak processionary caterpillar, dubbed the “hell larva” in Sweden, is raising concerns due to its spreading presence and toxic effects. The caterpillars’ tiny hairs contain a venom that causes skin irritation, respiratory problems, and allergic reactions upon contact. These hairs can detach and become airborne, spreading the risk far beyond the trees they inhabit. Authorities advise the public to avoid contact with the caterpillars and their nests, particularly during their active period. The venom settles on the body, exacerbating symptoms; medical attention should be sought if exposure occurs. Increased temperatures are believed to be contributing to the caterpillar’s range expansion, posing a growing threat to both human and animal health and forestry.