A bizarre chain of events in a college microbiology lab class left 33 people, including students, professors and lab workers, suspecting they were exposed to a deadly germ. Of those, 32 began taking an antibiotic regimen as a precaution to prevent life-threatening infection. Two also underwent lumbar punctures—spinal taps—after developing mild symptoms.

But none of them had actually been exposed to a dangerous pathogen. The incident was published in a recent issue of Morbidity and Mortality Weekly Report, offering an unusual cautionary tale for the ages.

The initial lab exercise was meant to be straightforward. Students were given mystery bacteria by their instructors to identify through biochemical tests and morphological characteristics. Unbeknownst to them, the mystery bacteria was supposed to be Plesiomonas shigelloides, a common non-scary bacterium, which infects animals and can cause mild diarrhea in humans.

However, to the instructors' great surprise, every single person in the class identified their mystery germ as Neisseria meningitidis. This dangerous pathogen can cause meningitis (swelling of the brain and spinal cord) and bloodstream infections. It spreads by aerosols and requires specialized handling measures for laboratory work, including respiratory protection and a biosafety cabinet to prevent exposure.

The instructors couldn’t find fault with their conclusion; all identified it as a gram-negative, oxidase- and catalase-positive diplococcus with carbohydrate fermentation patterns consistent with N. meningitidis. The students lacked the necessary protective equipment, only wearing gloves and lab coats.