“At best, you can get contact dermatitis. At worst, you can die,” said Jason J. Dombroskie, manager of the Cornell University Insect Collection and coordinator of the Insect Diagnostic Lab in Ithaca, N.Y. “You can go into anaphylactic shock and have your airways close up. The airborne hairs set up a whole different ballgame.”

British officials have issued similar warnings in years past as they have battled to stop the spread of the insect. This year, the Forestry Commission began treating trees in a “control zone” around the infected area with biopesticides, which use viruses or bacteria that mostly harm the target pest. The treatment is expected to continue through late May or early June, with trees at more than 600 sites expected to be targeted, the agency said.

“We advise people not to pick up the caterpillar or pick up the nest,” a spokeswoman for Britain’s Department for Environment, Food and Rural Affairs said, adding that there have been no reports of serious illness because of contact with the caterpillar.

Dr. Robert Glatter, an emergency doctor at Lenox Hill Hospital in Manhattan, said the speed of the reaction after contact is typically slower than that of a bee sting.

He said each caterpillar has more than 62,000 toxic hairs, which after release can stay active for up to five years. “This is where the true toxicity of the caterpillar arises,” he said, adding, “Word to the wise — it’s best to observe their beauty from a distance.”