Mr. Abri found a similar response among the homeless people he meets. “They know Covid-19 could be a threat,” he said, “but they are more worried about the City of Berkeley forcing them to move, like in the next day or two. So a potential virus the following week doesn’t feel so imminent.”

Other Americans, housed and free, live in overcrowded accommodations; in California, 13 percent of rental units hold an average of two or more people per room. And this figure excludes owner-occupied homes with people squeezing in extra family members, leading to the sort of close proximity that spread respiratory ailments.

The threat, though, is not just to the marginalized and medically underserved, but to everyone. Poorly paid, poorly housed workers prepare and deliver the food others buy from supermarkets and online retailers. Many clean public spaces to keep them germ-free for all. Many are fixing roofs, serving food to the elderly in nursing homes and the young in day care. Even if they develop a cough or fever, many continue working to feed their families, potentially exposing others to infection.

The rich may imagine that money itself is a vaccine, as they consult concierge doctors or zip in private jets to island vacation homes. But in highly unequal societies, such as ours, wealth is no protection from illness, according to a series of studies conducted by two British epidemiologists, Richard G. Wilkinson and Kate Pickett. Comparing the 25 most economically equal states in the United States to the 25 least equal, they found that at every level of income, mortality rates were higher in the unequal states.

There’s no point thinking you’re safe if those most vulnerable are not. Without wallet or passport, Covid-19 spreads across every class and circumstance with alarming ease. We’re in this together.

The societal immune system is much like nature’s immune system. When a virus enters the body, “first responder” cells alert neighboring cells. These neighboring cells communicate with more distant cells of different colors, origins and roles. They collaborate to contain the infection.

These days, we live with a paradox. Each time we touch a door knob, a subway pole, an elevator button or the hand of someone in need, we may feel a twinge of fear about our connection with others. But connection also saves us.