Visitors arriving from most states to the nation’s capital soon must self-quarantine for 14 days to help slow the increasing spread of the novel coronavirus.

D.C. Mayor Muriel E. Bowser (D) said her order, which takes effect Monday, will help the city battle a recent increase in coronavirus infections. It’s the latest action taken this week in the greater Washington region as leaders try to avoid the infection spikes seen elsewhere in the country.

“We know, unfortunately, there are states that are seeing significant spikes in new cases,” Bowser said. “We know there are places where people are not being as cautious or making the sacrifices we are making here in Washington, D.C.”

Bowser exempted Maryland and Virginia from the order, though both states meet the city’s criteria to be included. Virginia officials say the state isn’t considering any similar self-quarantine measures, while Maryland officials declined to say Friday whether such a measure has been discussed.

The District — which on Friday recorded its highest daily caseload since early June — defines a high-risk area as one that is seeing new daily cases exceed 10 per 100,000 residents. Bowser said her administration would post an updated list of states covered by her quarantine order every two weeks.

The city hasn’t yet published a list, but a similar analysis by The Washington Post shows only 16 states would likely be exempted from the mayor’s order.

The states that appear not to meet the standard for mandatory quarantine include Colorado, Connecticut, Hawaii, Maine, Massachusetts, Michigan, Montana, New York, New Hampshire, New Jersey, Oregon, Pennsylvania, Rhode Island, South Dakota, West Virginia and Wyoming.

The text of the mayor’s order does not specify whether the city could levy fines for those who break the self-quarantining rules. The mayor’s chief of staff said he did not know.

The D.C. Office of Planning sent the city’s universities and colleges a letter Friday, requiring them to demonstrate how they will help out-of-state students quarantine during their first two weeks on campus. The city’s health department can inspect the colleges’ lists of quarantining students for compliance.

The city recommended that colleges ask students to sign a behavioral pledge, agreeing to coronavirus safety measures such as wearing masks and limiting parties.

The Washington region’s increasing caseloads prompted New York Gov. Andrew M. Cuomo (D) this week to add Maryland and Virginia to his state’s travel advisory, which also requires visitors to quarantine for 14 days. Maryland Gov. Larry Hogan (R) has said he thinks New York “made a mistake” and that most metrics in his state other than caseloads are “trending pretty well.”

Christopher Rodriguez, D.C.’s director of homeland security and emergency management, said the Washington Nationals will be treated as “essential” under the city’s waiver for the team, even though sports are not otherwise on the mayor’s list of essential activities.

Team members and visiting opponents will be allowed to play baseball and participate in essential activities such as eating meals. Otherwise, players who have traveled to hot spots will have to stay home or in their hotels.

The team announced Thursday on Opening Day that outfielder Juan Soto had tested positive for the virus. Two players tested positive during intake screening at summer training earlier in the month, while another tested positive before arriving in the city.

Friday’s order to self-quarantine came in the same week D.C. expanded its mask requirements after noting a rise in the community spread of the virus.

Bowser announced Wednesday she would require people age 3 and older to wear a mask when they leave the house and are likely to come into contact with others. That order, which went into effect Thursday, indicated that violators could be fined up to $1,000, but city officials say few fines are expected.

The city concluded this week that it has seen increased community spread of the virus since late June, when it entered the second phase of loosening restrictions. That metric excludes the virus’s spread among residents of nursing homes and other facilities who are unlikely to transmit infections outside their buildings.

D.C. on Thursday reported the virus-related death of a 24-year-old woman. Her death came after city health officials voiced concerns that young people are making up a growing share of coronavirus cases.

In Maryland, two localities in the past week have restricted activities that initially were permitted under reopening plans.

Anne Arundel County capped the size of indoor gatherings at 25 and outdoor gatherings at 50, barred congregating inside shopping malls and ordered bars and restaurants to close by 10 p.m. daily. Baltimore officials are suspending indoor dining at bars and restaurants, beginning at 5 p.m. Friday.

D.C. on Friday recorded 78 new coronavirus cases and no deaths. The new cases brought the city’s seven-day average caseload to 76, its highest since June 8.

Maryland reported 930 new cases — the state’s highest daily caseload since May 30 — with most infections coming from Baltimore County, Prince George’s County and Baltimore City. Maryland also recorded 13 additional fatalities.

Virginia had 1,127 new cases Friday, the state’s highest daily number since June 7, which lifted its seven-day average caseload to 990. Nearly 500 of the new infections were reported in the Hampton Roads region, which has seen a surge that began earlier in the month. Virginia also reported 13 additional fatalities.

Antonio Olivo and Ovetta Wiggins contributed to this report.