Pastor Matt Doan of Calvary Church Santa Ana, in Southern California, pours grape juice into individual plastic cups, each about half the size of a shot glass. He fits them into deep silver trays, in preparation for the next day’s Communion.

During a Communion service, Christians re-enact the last meal Jesus shared with his disciples by eating a piece of bread and drinking a bit of wine or grape juice.

“It’s a joyful moment as well as kind of a reflective moment,” said Pastor Matt Doan of Calvary Church Santa Ana, in Southern California. “It’s one of my favorite parts of being a pastor and being part of a church community.”

Traditionally, participants walk to the front of the church and take a sip of wine or juice from the same chalice. The officiant wipes the lip of the cup and the process repeats for the next person in line. At Doan’s church, like many Protestant churches around the world, the Communion grape juice is instead served in individual plastic cups.

These cups are an artifact of a debate which started in the mid 1800s about the risks of catching a disease from a shared Communion cup.

For most of human history, people knew they could get sick from other people, but it wasn’t clear exactly how. During the 1860s, scientists in Europe landed on the idea that diseases are caused by germs. These microscopic, hostile agents enter your body from objects in your environment and make you sick. The germ theory was a breakthrough in the treatment of disease. It was suddenly clear: In order to go after disease, you have to go after germs.

“At the time the germ theory is introduced to your average American, the leading causes of death were still infectious diseases,” said Nancy Tomes, a historian of medicine and U.S. culture at Stonybrook University in New York.

“People paid attention because they didn’t want to die. They didn’t want to see their relatives die.”

A motley crew of physicians, political activists, entrepreneurs, and religious crusaders took it on themselves to go after germs. These “sanitarians” targeted kissing as a source of possible contamination, among other things. In an 1888 medical journal, one doctor went after “the poisoned chalice” — the shared Communion cup. “People were really easy to scare,” said Tomes.

But some of the sanitarians pushing churches to adopt individual cups had mixed motives. For example, some of the the sanitarians were preoccupied with the “moral cleanliness” of society, making pronouncements like “cleanliness is close to godliness.” Other sanitarians owned companies that sold individual Communion cups. James Buckley, the editor of a Methodist newspaper, writing in the 1890s, detected still another motivation for switching to individual cups: racism.

James Buckley had heard of congregations serving Communion to white people first whenever they happened to be in church with black people. And he observed individual cups would take this a step further by letting white Christians avoid drinking from the same cup as black Christians entirely.

“It’s not just African-Americans in the south, it’s also new immigrants who really get the brunt of this idea,” said Tomes. “You’re new, you’re coming in, and you don’t look like me, ergo, I’m going to assume the worst about your germ status. It’s just so easy to see this kind of protection as protecting yourself against the person rather than the microbe.”

By all accounts, writer James Buckley, who defended the common cup, seemed on board with the germ theory. Throughout his editorials, he was just asking scientists to prove the science behind individual cups without the racist element.

Eventually, James Buckley concluded for himself that the risk of infection from the common cup wasn’t significant enough to change a ritual defined by Jesus and upheld by Christians for thousands of years. Plus, he thought the traditional approach to Communion better embodied the kind of radical solidarity Christians should feel towards the poor and the marginalized.

Ultimately, lots of Protestant churches turned to individual Communion cups for different reasons. But James Buckley’s initial concerns that racism was operating in the background of some of these decisions kept resonating. Almost 100 years later, Martin Luther King Jr. famously described 11 a.m. on Sunday mornings as the most segregated hour in Christian America.

And what about James Buckley’s bacteriological predictions? Just how risky is it to share a Communion cup with someone else?

“Even though a person may be ill, and put some virus on there,” said Charles Gerba, an infectious disease specialist at the University of Arizona, “in the process of wiping it off, and maybe some of the anti-microbial activity of the silver in these cups, that seems to largely reduce the risk.”

In other words, you’re more likely to contract germs on the door handle leading into church or shaking someone’s hand after the service than on the lip of a Communion cup.