Baseball is played outdoors, and there is little physical contact compared with the frequent full-body collisions of football and basketball. In theory, players are at relatively low risk of spreading COVID-19.

Yet, after 13 members of the Miami Marlins organization tested positive for the virus, the team opted to quarantine in Philadelphia after its Sunday game with the Phillies. And the Phillies postponed their Monday game with the Yankees while the mess is sorted out.

Is there a safe way to proceed with pro sports?

Physicians and infectious disease experts say that is a challenging question, as the word safe suggests a yes-or-no dividing line when the reality involves balancing degrees of relative risk.

For baseball, the actual game may involve a lower risk of spreading illness than some other sports, but league officials opted for a higher level of risk outside the ballpark. Unlike the “bubble” approach of the NBA, with teams confined to a central location, baseball teams are traveling from city to city.

That increases exposure for players, hotel workers, and other support staff, and makes it harder for teams to enforce precautions such as mask-wearing and social distancing, said Charles Cairns, dean of the Drexel University College of Medicine.

“You can’t really enforce compliance when you’re not in a bubble,” he said. “You bring the wild cards of exposure to travel and people who aren’t part of that system.”

Yet a baseball bubble would be a challenge, given that teams are larger than in basketball and require bigger venues, said epidemiologist Joseph Amon, a professor at Drexel’s Dornsife School of Public Health.

Other measures might allow the league to reduce risk to an acceptable level, he and Cairns agreed. Traveling to games in separate cars, perhaps. Allowing for more space between players in dugouts and bullpens. Increased testing and notification of others who are exposed.

Football will be harder, Amon said.

“Think about linebackers lining up against each other, breathing into each other’s faces,” he said. “You breathe hard when you exercise.”

Epidemiologists say there are three key terms to keep in mind for minimizing transmission of the coronavirus and other respiratory viruses: place, space, and time. Generally, exposure to an infected person is thought to be riskier indoors than outdoors. The risk is higher when the infected person is less than six feet away because the virus-laden droplets are less likely to travel beyond that range. And the longer the duration of exposure, the greater the risk of transmission.

Many football stadiums are outside. But the sport fails the six-foot threshold. A bubble is a must for the NFL, Arthur Caplan, a prominent medical ethicist at the New York University School of Medicine, said on Twitter.

“No bubble, no chance,” he tweeted.

Logistically, it would be hard. Meanwhile, with baseball, the Philadelphia Department of Public Health has been working closely with both the Phillies and Marlins to minimize the risk of additional spread, agency spokesperson James Garrow said.

The Marlins are helping to identify and notify all people who may have been exposed to the infected players so they can take proper precautions, he said.

Garrow issued a statement from the department as follows:

”We support the decision by the Marlins to shelter in Philadelphia in response to the positive tests. Members of the Marlins organization who have tested positive are in isolation in the team hotel, which is taking all of the recommended precautions to protect staff, including no-touch food delivery.”

The risk to others outside the hotel and ballpark is “extremely low,” the agency said.