It was not surprising when three-quarters of the house tested positive. There were 12 people in three bedrooms, with a bathroom whose door frequently required a knock and a kitchen where dinnertime shifts extended from 5 p.m. well into the evening.

Karla Lorenzo, a Guatemalan immigrant who cleaned houses in San Francisco and Silicon Valley, lived in the big room along the driveway. Big is a relative term when a room has five people in it. She and her partner, Abel, slept in a queen-size bed along the wall. There was a crib for the baby at the foot, with the older childrens bunk bed next to that. The other housemates had similar layouts.

Living among many people, as Ms. Lorenzo put it in Spanish, you cannot really avoid your housemates. The sounds, the smells, the moods everyone is pressed against all of it, and they understood that if one of them got the coronavirus, the rest probably would.

That happened in April, and now the house is returning to health. Abel, referred to by his first name because his immigration status is uncertain, is home after three weeks in the hospital, where Ms. Lorenzo feared he would die alone gasping for air. And she is no longer squirreled in the closet where she spent days to avoid giving the virus to the children.

Now comes a second struggle: figuring out how to pay rent. Abel is back at work at a home supply store, but Ms. Lorenzos housecleaning jobs dried up and one of the other families moved out increasing the monthly bill by $850. We dont know how we are going to do it, she said.

From the early outbreaks to the economic destruction that has come after, the coronavirus pandemic has mapped itself onto Americas longstanding affordable housing problem and the gaping inequality that underlies it. To offset rising rents in a nation where one in four tenant households spend more than half of their pretax income on shelter, a multitude of low-wage service workers have piled into ever more crowded homes.

Living in overstuffed units subdivided by hinged partitions and tacked-up sheets, these households many of them retail and service workers who are unable to do their jobs from home were acutely susceptible to the viruss spread. With double-digit unemployment projected to persist through next year, the same families face losing the crowded homes that make it so easy to get sick in the first place.

To combat the virus, Americans of every income are being encouraged to wear masks and keep their distance. But for low-income families who crowd together to stretch their budgets, home has its own risks.

For these families, a good amount of the response has included triaging a decades-old shortage of affordable housing. Cities and states are renting hotel rooms for people who normally sleep on the streets. There are trailers to quarantine those whose apartments are too crowded for isolation. Fearing a wave of homelessness, governments have followed up with rental aid and moratoriums on evictions.

Combined with federal stimulus funds, and $600 a week in supplemental unemployment benefits that have just lapsed, these measures have prevented the dire predictions of mass displacement. Congress is working on another emergency package, and property owners and affordable-housing advocates have pressed for direct rental assistance.

But evictions are already ramping back up, and the longer the economic malaise continues, the more housing insecurity there will be. Some of the evicted will become homeless, but if the past is a guide, most are likely to find somewhere else to go, and that somewhere is likely to be overcrowded compounding the conditions that make it so easy to spread the virus.

We have clients struggling to choose between living in an overcrowded home or facing eviction for not being able to make rent, said Nazanin Salehi, a lawyer with the nonprofit group Community Legal Services in East Palo Alto. No matter what they decide, the risk is more exposure to this virus.

Visitors to Silicon Valley may take a wrong turn or freeway exit on the way to this or that office park and find themselves in an area like the North Central neighborhood of San Mateo, Calif. That is where Ms. Lorenzo lives on a block of faded homes on small lots, with packed driveways and cars parked liberally on the sidewalk. The scene is one side of the tech economy.

For much of the peninsula stretching south from San Francisco, there is a rough economic split. Cities and neighborhoods to the east, places like East Palo Alto, North Fair Oaks and the Belle Haven section of Menlo Park, are more overcrowded and have a larger share of low-income and Black and Latino residents, many of whom have been disproportionately affected by the virus. Towns and neighborhoods to the west, places like Hillsborough and Palo Alto, are whiter and rich.

This geography is as fundamental to how the place operates as the invention of the microchip. Every day, throngs of clerks, landscapers and elder-care workers wake up on the eastern parts and travel to homes on the western parts or to the corporate campuses of tech companies to do subcontracting work. And every night, they return to overcrowded homes.

Ms. Lorenzo was one of them. She immigrated to the United States six years ago from Guatemala with her two children, fleeing a broken relationship and looking for a new start. Now she is a green-card holder with a new partner and a 2-year-old. Until the pandemic hit, she made about $16 an hour mopping floors and vacuuming carpets in homes on the other side of the peninsula.

For a while, her wages and Abels were enough for their own small place a $1,600-a-month studio that had a bed for them and a shared mattress for the children. Then the rent jumped to $2,100. And then to $2,650.

The couple went looking for cheaper housing and roommates, a quest that has become a Bay Area ritual. Since the Great Recession, a growing share of Bay Area movers, from all but the most well-off households, have gone to homes with four or more adults from ones with one or two adults, according to a study by researchers at Stanford University and the Federal Reserve Bank of San Francisco.

The high-end version is dressed up with a description like co-living or explained as a culturally in-tune couple sacrificing an extra bedroom in the suburbs for a life of less driving closer to the city. The low-end version is poverty. Whatever it is called, the economic calculus is the same.

Wages are higher in coastal California than in inland areas, where housing is cheaper, so all but the very rich have to make a trade-off between a commute and space. It is just that the choices for poorer workers are more extreme, like a three-hour commute from cities like Stockton or huddling together in homes where nearly every space is the site of someones bed.

Researchers define extreme overcrowding as any home that is occupied by more than one person for every room without a toilet. By this measurement, overcrowding has increased nationwide since the mid-2000s, and the problem is particularly acute in California. About 13.4 percent of rental units more than double the national average were considered overcrowded in 2018, according to the Census Bureau. San Mateo and Santa Clara Counties, which roughly outline Silicon Valley, have one of the worlds densest concentrations of billionaires as well as some of the countrys most overcrowded homes.

After the studio, Ms. Lorenzo found a $1,250-a-month room in her current home, a blue stucco house at the back of a two-unit lot, with chalk drawings on the driveway and a dirt yard in the back. There were 11 occupants after Ms. Lorenzo moved in, 12 after her younger child was born.

Dividing the rent had benefits, like allowing Ms. Lorenzo to save money and buy her first television. The childrens shared mattress from the studio was replaced with a new bunk bed. More clothes, more shoes for the children, she said, because we were limited in many things.

The catch was living with personalities, rules and understandings. Cooking privileges were on a first-come basis, which meant that the last family to use the kitchen might not eat until 9:30 p.m. There was no official time limit on the bathroom, but people knew to be fast. If anyone got a cold, everyone was exposed.

Crowded homes have been a concern practically as long as public health has been a field. Living with a pile of roommates has long been associated with faster-spreading infections, inescapable stress, irregular sleep and the effects that follow, including higher blood pressure and weakened immune systems.

But those take years to develop. The coronavirus spreads in days. By moving so fast and furiously, the virus has exposed in weeks something doctors have been worried about for generations, said Dr. Margot Kushel, an internist and director of the Benioff Homelessness and Housing Initiative at the University of California, San Francisco. Covid has really become a story of essential workers living in crowded housing, she said.

The sickness began, as it does, with worry.

In mid-April, after schools shut down and the children were sent home with worksheets, Abel returned from his job with a report that two of his co-workers had been out sick. He showered with the garden hose and slept in the car that night. But it was too late.

His symptoms were initially mild, before escalating to a 104-degree fever and a shortness of breath that prompted Ms. Lorenzo to take him to the hospital. The county health department, worried that a crowded home would accelerate the spread of what was confirmed to be the coronavirus, dispatched a case worker to test everyone in the house, Ms. Lorenzo said. Eight all except her children were also positive.

Ms. Lorenzo never got more than a headache and a sore throat, which in normal times would not have even prevented her from going to work. Suddenly she had to isolate herself in a house where everything was shared.

She settled on the closet, running a phone charger under the door and sitting there for six to eight hours a day, playing word games on her phone, calling relatives in Guatemala, sometimes just napping. Her 10-year-old son took over cooking meals and changing diapers. All the while, Abel was in the hospital. Improving or worsening, alive or dead, Ms. Lorenzo had no idea.

There was no communication with him, so my head was spinning, she said.

Ms. Lorenzo sprayed down the bathroom whenever she or the children used it. She avoided the kitchen and had her sister, who lives more than a half-hour away in Oakland, deliver food through the bedroom window. One time, the sister brought a thermos of hot coffee that Ms. Lorenzo said might as well have been hot water; the virus had so ruined her sense of taste that she could not tell the difference.

Still, the house got tense. One of the housemates accused Abel of infecting them. She told Ms. Lorenzo that if anyone in her family died, she would figure out a way to sue her. After that came the silent treatment no hablaba and as house relations plummeted, Ms. Lorenzo feared she would be evicted with nowhere to go.

After two weeks, a county health worker returned to test the house again. Ms. Lorenzos children were still negative, which seemed so unlikely, given the crowding, that the county retested them several times. All negative, she said. Worried that this luck would soon run out, the county moved her and the children to an emergency trailer.

They lived there for nine days, leaving only to collect stale salad and sandwiches left on an outdoor table. When they finally went home, Abel was back from the hospital.

Days of deep cleaning ensued. Ms. Lorenzo, back to health, is wondering when the world will return to some semblance of normality. Yet she feels lucky that things are not worse, because she thought her partner was going to die. We are trying to cope with it, she said. Trying to leave everything in the past.

Early in the outbreak, Gov. Andrew M. Cuomo of New York and some commentators blamed dense housing and public transit for the spread of the virus. The proof seemed as intuitive as New Yorks status as an early epicenter. The recent surge of cases in the more sprawling metropolitan areas of the South and the West has undercut that thesis, and a number of new studies suggest that density, the number of housing units per acre, is less important than crowding, the number of people per bedroom.

One widely cited report was from New York Universitys Furman Center, which found that infections were much more intense in Queens neighborhoods with high rates of overcrowding than in Manhattan neighborhoods with higher density but fewer people per unit. The link between crowding and transmission has since shown up in suburbs, rural America and Native American reservations. There is even some evidence that dense metropolitan counties, while suffering higher raw numbers of infections, have a lower death rate because it is easier to get to a hospital.

San Mateo County has been a bright spot, with a rate of about 700 coronavirus cases per 100,000, about half the rate of the state. Still, the countys cases have been concentrated in low-income households, with most coming lately from front-line workers who live in crowded multigenerational conditions, according to the county health officer.

In Chelsea, Mass., which had one of the nations worst outbreaks, there is a compelling suggestion that less-crowded quarters can help control the spread. Sleeved into the same blocks where buildings were overrun with infection are 375 subsidized apartments owned by The Neighborhood Developers, a housing nonprofit. The 968 tenants are mostly nonwhite, have the same mix of low-paid service jobs as their neighbors, and live in multistory buildings. But their units are subsidized and less crowded and so far, healthier.

The Neighborhood Developers has had eight reported cases of the coronavirus in Chelsea, or 826 per 100,000 people, about a tenth the rate of the surrounding community. Its not how many people you run into on the street but how many people you see when you come home, said Rafael Mares, executive director of The Neighborhood Developers.

The story is tempered by its rarity. The United States has a deficit of seven million apartments available to the lowest-income households, or an average of 36 available affordable units for every 100 extremely low-income family in search of one, according to the National Low Income Housing Coalition.

In April of last year, The Neighborhood Developers opened a five-story building with 34 apartments for homeless and low-income families. It received 3,598 applications.

Stacked against a wall in Ms. Lorenzos living room are three red-and-white coolers that her sister used to fill with ice cream to sell on the street. They are furloughed because of the lack of demand and have become just another obstacle that her cooped-up children have to dodge while zipping around the house.

Abel still gets headaches and a tremor in his left arm, but the virus is gone and he is well enough to work. Ms. Lorenzo has not cleaned a house since March but recently got a new job cleaning offices. The family has also been relying on nonprofit organizations and Christian charities for staples.

Once a week Ms. Lorenzo joins the procession of cars that roll through a parking-lot food bank set up by Samaritan House, a San Mateo-based organization that has seen demand for food double and is spending $200,000 a week on rental assistance. Since April 1, 4,000 families have applied for some $8 million in assistance on rent and utilities and it hasnt even really hit yet, said Bart Charlow, Samaritan Houses chief executive.

Ms. Lorenzos name could soon be on the list. In June, the departure of the angry housemates opened up an extra bedroom, and her family spread out, with the older children moving across the hall the sort of arrangement that the San Mateo County Health Department has been recommending for years, except that it is financially unsustainable.

After taking the extra bedroom, Ms. Lorenzos familys share of the rent jumped to $2,100 from $1,250. Their savings got them through July. Now that money is gone, and August is here.

Liliana Michelena and Ben Casselman contributed reporting.

More:

Overcrowded Housing Invites Covid-19, Even in Silicon Valley - The New York Times