Public health crises have spawned conspiracy theories as far back as when the Black Death ravaged Europe in the 1300s, as people desperately try to make sense of the chaotic forces disrupting their lives. While modern science offers a better understanding of how diseases infect people and how to contain them, COVID-19 conspiracy theories are spreading rapidly via social media, unreliable news outlets and from our own political leaders, including U.S. President Donald Trump. The result: many Americans now believe pandemic-related conspiracy theories—and, alarmingly, those same people are less likely to take steps to prevent the virus from spreading.

In a University of Pennsylvania Annenberg Public Policy Center study published Monday in Social Science & Medicine, researchers surveyed a group of 840 U.S. adults—first in late March, and then again in mid-July—to determine how Americans’ beliefs and actions regarding the pandemic changed over time. Overall, they found that COVID-19 conspiracy theories are not only commonplace, they’re gaining traction. Back in March, 28% of people believed a debunked rumor that the Chinese government created the coronavirus as a bioweapon; that number rose to 37% by July. About 24% believed that the U.S. Centers for Disease Control and Prevention exaggerated the virus’ danger to hurt Trump politically despite a lack of evidence; by July, that figure rose to 32%. And in March, about 15% of respondents said they believed that the pharmaceutical industry created the virus to boost drug and vaccine sales—another unfounded theory—compared to 17% in July.