Authorities are closing honky tonks, bars and other drinking establishments in some parts of the U.S. to stem the surge of COVID-19 infections — a move backed by sound science about risk factors that go beyond wearing or not wearing masks.

In the words of one study, it comes down to the danger of "heavy breathing in close proximity."

Crowded indoor spaces filled with people yelling, leaning close to hear one another and touching the same sticky surfaces are "the opposite of social distancing," said Dr. David Hamer of the Boston University School of Medicine.

"Can you do social distancing at a bar? Can you wear a mask while drinking?" Hamer said. "Bars are the perfect place to break all those rules."

The rapid spread of a bar outbreak can swamp public health workers. In East Lansing, Michigan, an outbreak tied to a large brewpub near Michigan State University has spread to nearly 140 people in 12 counties, causing authorities to recruit nursing students and retirees to help with contact tracing.

"In 12 days, we went from two identified cases to 128, and, honestly, I don't have today's numbers yet," Ingham County health officer Linda Vail said Wednesday before cases shot up again. She described her outlook as "shocked and overwhelmed."

Michigan Gov. Gretchen Whitmer responded by closing indoor seating in bars in parts of the state. Taverns will not have to close completely. They can sell to-go cocktails and keep outdoor patios open.

In California, Gov. Gavin Newsom took similar action, ordering bars and indoor restaurant dining to close again for the next three weeks in most of the state. New York Mayor Bill de Blasio delayed the city's resumption of indoor dining.

Two other factors at play in bars make them potential virus flashpoints. Alcohol lowers inhibitions, so people forget precautions, said Natalie Dean, an infectious diseases expert at the University of Florida.

Plus, the attractive, healthy person buying you a drink could be a silent carrier, shedding contagious virus with each breath.

"Young people have less severe illness, so they may be infected and able to infect others inadvertently," Dean said, noting outbreaks in Japan and South Korea associated with restaurants, bars and karaoke parties.

In recent weeks, college towns across America have seen clusters of cases that have been traced back to bars. Bars and restaurants near the University of Iowa and Iowa State University closed only weeks after the governor allowed them to reopen.

As of last week, 90% of cases in the county that is home to Kansas State University involved people ages 18 to 24. Health officials said most of them spent time in a bar and restaurant district known as "Aggieville."

Citing a similar spike, the Kansas county that includes the city of Lawrence and the University of Kansas also ordered bars and nightclubs to close beginning Friday for the next two weeks.

"Congregation at a bar, inside, is bad news," Dr. Anthony Fauci told a Senate panel Tuesday. "We really have got to stop that."

Texas, Arizona, Los Angeles and some Pennsylvania counties are closing bars to slow the spread of the virus. Florida and Colorado have told bars they cannot serve alcohol on site.

Most bar owners and employees feel that they have been unfairly singled out, particularly because restaurants are still open and serving alcohol. In Texas, bar owners said that on Friday after they were forced to close their doors, they noticed restaurants were still packed.

"You can go into a restaurant and they have bars, and you can have as many drinks as you want," said Nikki Forsberg, owner of the Old Ironhorse Saloon in Blanco, Texas. "It doesn't seem fair. Restaurants get this pass and the bars don't."

The bar's manager, Tami Cooley, said although she did not wear a mask at work, she felt the tavern was taking every precaution to safely stay open. No one was allowed to drink or order at the bar, and tables were limited to six people. They closed for a few days after finding out one of the bartenders had been in contact with someone who tested positive for the virus.

"We were cautious at our bar. We social-distanced, sanitized the tables, chairs, bathrooms, doorknobs," she said. "We did everything right."

In the Michigan bar outbreak investigation, "huge concern" now centers on the parents and grandparents exposed to the virus by their offspring who partied at the brewpub, Vail said. She worries about "a climb in secondary cases if the people we asked to self-quarantine didn't do that ... Where is this secondary transmission going to land?"

Saskia Popescu, an infectious diseases expert in Phoenix, said it's difficult to disinfect surfaces at a bar enough to make a difference. Even sitting at a table with friends at a bar involves loud talking and laughing that could spread virus. It's not worth it, she concludes.

"You can make a cocktail at home," Popescu said.

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Associated Press Writer John Seewer in Toledo, Ohio, contributed to this report.

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The Associated Press Health and Science Department receives support from the Howard Hughes Medical Institute's Department of Science Education. The AP is solely responsible for all content.