Prediction market speculators have bet millions on the next US measles outbreak, potentially helping public health systems track the disease’s spread.

The markets sell shares in outcomes, which pay out if they happen; the idea is to harness the wisdom of the crowd. They have successfully done so in disease prediction, and while “the ethics … are murky,” often outperform epidemiological modelling, New Scientist reported.

The markets gained particular attention after gamblers predicted the Iranian supreme leader’s assassination and a soldier was arrested for insider trading on the Venezuelan president’s capture: Foreign leaders “should probably be checking their removal probability on Polymarket every few minutes,” Bloomberg’s Matt Levine noted, in case “it’s someone on the helicopter getting in one last trade.”