One year ago, a report from the Johns Hopkins Center for Health Security assessed the readiness of 195 countries to confront a deadly disease outbreak. Topping the list of most-prepared nations was the U.S.

But that forecast didn’t account for one crucial factor: the toxic degree of partisanship that would turn something as simple as wearing a face mask into a political statement.

How did things get so bad that Americans couldn’t come together to confront a universal threat like COVID-19?

A new report published in Science offers an explanation — political sectarianism. Political sectarianism goes beyond mere disagreements.

What pushes mere enmity into the realm of political sectarianism is a “poisonous cocktail” of beliefs that turns opponents into mortal enemies regardless of the issue, according to the 15 experts in political science, social psychology, sociology and cognitive science who co-wrote the report.

This cocktail has three key ingredients, they said. The first is “othering,” which is a “tendency to view opposing partisans as essentially different or alien to oneself.”

The second is aversion, a reflex to “dislike and distrust” one’s political opponents.

The final ingredient is moralization, which causes us to see our opponents as not merely wrongheaded, but evil.

“It is the confluence of these ingredients that makes sectarianism so corrosive,” they wrote. “When all three converge, political losses can feel like existential threats that must be averted — whatever the cost.”

The effects of political sectarianism can be seen with something social scientists call a “feeling thermometer.” It’s a scale that puts cold feelings at 0 degrees and warm feelings at 100 degrees. If you feel neutral toward someone, they measure 50 degrees.

Over the past 40 years, Americans’ feelings toward members of their own political group have remained relatively stable, around 70 to 75 degrees, said the American National Election Study.

But when it comes to feelings for those in the other party, opponents registered at 48 degrees in the 1970s and about 20 degrees now. In other words, we now hate our opponents more than we love our allies.

In this environment, politicians have little to gain — and much to lose — by trying to find common ground with the other side. “Issues that are not inherently partisan become politicized,” the authors write. A case in point: the decision about whether to wear a mask during the pandemic.

Masks have come to be associated with Democrats, making Republicans less inclined to wear them. “The result has been lethal and expensive for Americans across the political spectrum,” the report said. “Political sectarianism cripples a nation’s ability to confront challenges.”