Gabriel Santos, a homeless-outreach worker, had woken up with cold symptoms. Not wanting to expose his clients to illness, he reached for the mask. It would be better this way, he thought. But now, as he approached the woman sheltering outside a Maryland building, its other effects became clear.

“Don’t come over here with no mask on,” shouted Debra, a chronically homeless woman who sat bundled in a winter coat, a black-and-white blanket pulled over her lap. “I said don’t you come over here!”

Santos pulled the mask down, hoping she would recognize his face. A senior outreach coordinator with the nonprofit Bethesda Cares, Santos sees her often on his morning patrols of downtown Silver Spring, Md. But it was no use.

“I don’t care who you are!” she shouted again. “Don’t come over here with no mask!”

News of the rapidly spreading coronavirus, which as of Thursday had killed more than 170 people in the United States, has reached America’s homeless. But protocols meant to keep the public safe may do little to protect those without homes to shelter in — and could expose some of society’s most vulnerable people to even greater risk.

From San Francisco to Chicago to the District, best practices are meeting harsh realities in a patchwork of responses that vary state by state, city by city.

Several local and state governments this week have ordered restaurants, coffee shops, libraries and other public gathering places to close — cutting off access to places where homeless people on the street can go to use the bathroom and wash their hands.

Recommendations from the Centers for Disease Control and Prevention say to limit gatherings to 10 people and practice “social distancing,” keeping at least six feet from others. Implementing such guidelines at an emergency shelter meant to house dozens of homeless adults can be impossible.

Covid-19, the disease caused by the coronavirus, hits people over 60 and those with preexisting health conditions hardest. The virus is easily spread through touch and by lack of proper hygiene.

People experiencing homelessness are increasingly older and sicker. Many have underlying health conditions but lack access to primary-care physicians or preventive health screenings. They struggle to find public bathrooms to maintain basic hygiene. Those who live in tent encampments or crowded shelters might be unable to keep their distance from others or self-isolate if they show symptoms.

Even their fiercest advocates — experts who have long served the homeless in their communities — worry about inadvertently putting their clients’ health at risk by simply showing up and doing their jobs.

“The education we’re giving our staff going out and working with this population is they’re not the ones you have to worry about being a possible carrier of this virus right now — you are,” said Betsy Bowman, director for adult and community services at EveryMind, a nonprofit organization that works with the homeless in Montgomery County. “You’re the one who went to church and saw your friend who just got back from overseas. It’s more likely that you’re going to give it to them. And if an infection gets into that population, it’s going to be devastating.”

Millions of U.S. grandparents care for young kids — and are high risk for covid-19

The U.S. Interagency Council on Homelessness estimates there are 567,700 homeless people in the country. The most underserved live in parts of the District, California, Maine, Oregon and Washington state, according to a Washington Post analysis of data from the U.S. Health Resources and Services Administration.

In the nation’s capital, homeless shelters announced they would stay open around the clock, instead of the usual 12 hours, and emergency measures will allow Mayor Muriel E. Bowser (D) to temporarily house homeless families for up to 60 days.

Bowser said city officials are working to ensure any homeless person showing symptoms gets appropriate treatment — but what that looks like in practice is not clear.

“If we have a person who is experiencing homelessness who needs to quarantine, we will make sure this happens,” she said.

Wayne Turnage, the city’s deputy mayor for health and human services, said homeless residents are being given hand sanitizer while encampment cleanups — which typically force residents to move their tents before streets are power-washed — have been limited to trash pickups.

Those who stay in District shelters are being screened before entering, and residents are required to stay in the same bed in the same shelter to limit movement between facilities. The District’s guidance for homeless-service providers includes checking clients for fever, encouraging hand-washing, staggering bathing and meal times, and moving beds at least three feet apart.

Laura Green Zeilinger, director of the D.C. Department of Human Services, said it’s not known whether anyone in the District’s homeless population has tested positive because the city doesn’t report results based on housing status. She said the city has found space where up to 200 people could self-quarantine if they’re otherwise unable.

Shelter environments do not naturally lend themselves to social distancing behaviors, experts say.

In some shelters, people share rooms and sleep in bunk beds. In others, mats line the floor of empty rooms to squeeze in as many people as possible during the cold winter months. Meals are served and consumed in large cafeterias. Recreation rooms, bathrooms and laundry facilities are shared.

Many shelters do not have room to isolate someone for an extended period. Some already are running low on cleaning supplies, hand sanitizer, food and volunteers to relieve overworked staff. Others have told volunteers to stay home — an effort to limit the number of people in and out of shelters at a time when experts say even asymptomatic people can spread the coronavirus.

“Our homeless services here are countercyclical, so when all these services are shutting down, we have to be in ramp-up mode,” said Shannon Steene, executive director of Carpenter’s Shelter in Alexandria, Va., which serves families with children. “The thought of a potential self-quarantine with everyone in the shelter at one time, that is very sobering for me.”

The homeless are exempt from a public health order to “shelter in place” in seven Northern California counties, but officials encouraged them to seek shelter. For many of that region’s most vulnerable residents, it isn’t an option.

The San Francisco Bay area, home to some of the biggest names in the technology sector, also struggles with one of the biggest homelessness crises in the country.

There were more than 8,000 homeless people in San Francisco last year, according to city estimates based on a single night. By other city measurements, the number is more than 17,000.

“When you layer on top of that the coronavirus and mass vulnerability, we’ve moved beyond crisis to something bigger,” said Karen Hanrahan, chief executive of the Glide Foundation, a nonprofit that provides services to the homeless.

San Francisco Mayor London Breed (D) announced a $5 million emergency fund and public health order earlier this month to help vulnerable populations, including the homeless. The money will pay for roving cleaning crews to minimize contagion and support expanded daytime hours at shelters.

Local outreach teams run by nonprofits and the city are offering hand sanitizer and information on how to stay healthy. San Francisco also has deployed additional hand-washing stations on the streets.

>The city plans to offer temporary housing to help people exposed to covid-19 who can’t self-isolate, including dozens of recreational vehicles placed in neighborhoods. In its ongoing search for quarantine housing, the city is also looking at unoccupied residential properties and vacant hotel rooms.

Sarah Owens, a spokeswoman for Breed, declined to say how many additional spaces are still needed or whether testing for coronavirus is being done in vulnerable populations, citing patient privacy.

Jennifer Friedenbach, executive director of the nonprofit Coalition on Homelessness, estimated there are about 40,000 people in San Francisco who cannot self-quarantine, because their housing situation does not allow it, including people living on the streets or in shelters, jails or single-room-occupancy hotels.

In a region where many homeless people sleep in tents beneath overpasses, on sidewalks and in parks, Friedenbach said the city’s effort to protect vulnerable populations must include a moratorium on removing tents from encampments.

“I feel like the city is working really hard and trying to come up with stuff as fast as they can,” Friedenbach said. “I don’t think $5 million is going to get us to the place where we’re preventing the spread.”

In the Seattle area, where the first domestic coronavirus case was reported, care providers have been working on social distancing in shelters. Seattle city officials also are funding emergency hotel rooms for the homeless, said Daniel Malone, executive director of the Downtown Emergency Service Center, a homeless outreach organization.

Because testing for the coronavirus remains limited, city spokesman Scott Thomsen, Seattle has focused on spreading its homeless population out as much as possible.

No cases of coronavirus have been reported among Seattle’s homeless, said Malone, who added that the city was in need of additional tests for the homeless.

Tests have remained hard to get, even for those with health insurance and demonstrable symptoms. Shelters and homeless advocacy organizations said they are waiting — just like everyone else — for test kits to be more widely available.

“They’re asking me: ‘What if I want to get tested? Where do I go?’ ” said Kasia Shaw, a nurse practitioner with Arlington Homeless Services Center in Virginia. “I tell them: ‘Well, there are not enough tests. You’re more than welcome to call the emergency room, but they’re going to ask you screening questions, and if you don’t meet the criteria, then they’re just going to tell you to self-isolate.’ ”

Doug Schenkelberg, executive director of the Chicago Coalition for the Homeless, said the homeless population in the Windy City has been “way undertested.” He said a large proportion of Chicago’s homeless live not on the street but in small apartments with many other people.

“These are folks that don’t have stable housing, likely in crowded environments,” he said. “How do you provide support to them? They’re not as easily found as folks on the street or in the shelter system.”

To many advocates for the homeless, the North Star in this health crisis remains the same as it ever was — finding people stable housing.

Michael Clark, 35, has been staying at the men’s shelter in Montgomery County for three months. Though he is taking warnings about the coronavirus seriously, he said he feels as if it is a distraction from his most immediate needs.

“My goal here is to get my life on track,” Clark said. “If it’s not helping me move forward, then I just can’t have anything to do with it.”

Many shelters have begun to formulate no-contact supply drops and lunch donations. Workers and volunteers who assist the homeless are also increasingly anxious about their own safety.

“We have staff coming to work and handling business as usual, but we have volunteers saying, ‘I’m not sure I want to be there. It’s going to be more than 10 people,’ which is true,” said Pam Michell, executive director of New Hope Housing in Northern Virginia. “ ‘It’s really hard to be six feet away,’ which is true.”

Some homeless communities have started to self-police.

Shelter workers said grown men are following one another out of bathroom stalls, shouting to one another to wash their hands. Clients are staggering meal times and taking smoke breaks farther apart.

The District’s largest homeless encampments are centered around Union Station in the NoMa neighborhood, where people huddle in tents under two railroad underpasses.

Michele Hydier and her partner often sleep beneath the underpass after a day of panhandling. Business has been slow — on a recent day, she earned $1 and a fruit cup instead of the $20 she normally takes in.

Many NoMa stores that encampment residents rely on for bathrooms, for Internet access and to charge their phones are closed. NoMa residents are still leaving food and supplies for those in the encampments — just less of it.

“It’s like people are scared of us,” Hydier said.

Back in Silver Spring, five miles from the National Institutes of Health, a team of street outreach workers were handing out supplies — socks, snacks, a poncho. They did not have hand sanitizer to give. They were out of bottled water.

>When Santos, the homeless-outreach worker, reached to hand Debra a bar of soap, she pushed it away.

“What the hell am I going to do with that?” she asked.

“I know they’re closing things down,” said Deborah Taylor, programs director at Bethesda Cares. “I want to make sure you have somewhere to go, to use the bathroom, wash your hands.”

The woman shrugged, her face shrouded by an oversize furry hood.

“I probably don’t,” she said.

Tiku reported from San Francisco. Darran Simon and John D. Harden in Washington contributed to this report.