It is a variant of a disease that can go from a fever and headache to a galloping rash and then to death within hours — so quickly that some victims have been found dead in bed before they could even get to a doctor. Over the last two years, it has appeared only among men, and they often got it, health officials say, through anonymous sexual encounters with other men found through Internet chat rooms or digital apps or at parties, making it all but impossible to trace the path of infection.

It is a unique strain of bacterial meningitis, so new it has not even been named, and it is particularly lethal — killing one out of three people instead of the one out of five who succumb to other strains of meningitis.

New York City health officials are growing increasingly worried that this strain of meningitis, which is an inflammation of the lining around the brain and the spinal cord, is so insidious that it could suddenly mushroom into a major outbreak, claiming many lives before anything can be done to stop it.

“It’s been sort of marching through the community in a way that makes us very scared,” Dr. Jay Varma, the deputy commissioner for disease control at the city’s health department, said on Thursday.