It wasn’t just that people were getting sick—it was who. And how many.

Hepatitis A is a viral disease that primarily attacks the liver, and if it gets serious—as it can in the elderly and immune-compromised people—it can be fatal. But the graph of cases in the US over time looks like the second, fun half of a roller coaster ride. In the early 1970s, nearly 10,000 people a year got it. By the mid-1980s, the number was half that. (Wheee!) In 1996, the Centers for Disease Control and Prevention started recommending vaccination, and from there it was a fast, bracing plunge to just tens of cases a year.

Then, this past summer, people started getting sick in San Diego. Just a handful at first, among those most at risk. Like HIV, hep A gets transmitted through sex and sharing needles. You can also get it through fecal-oral contact—as can happen when people don’t have access to bathrooms. In San Diego, the infected were primarily homeless, illicit drug users, and men who have sex with men. The initial handful became two handfuls, and then the curve headed upward. Now, a few months later, the toll stands at 546 cases and 20 deaths, with a confirmed spread of another several dozen in Los Angeles and Santa Cruz.

Homeless people present a particular challenge for health—but for reasons as much political as medical. When the urban infrastructure shows signs of weakness, as it has with these hep A outbreaks, it’s not just a medical tragedy. It’s a signal of a failure yet to come. If social policy doesn’t deal with America’s ongoing social and political homelessness crisis, it’s going to be an even worse public health problem later—for everyone.

San Diego and Michigan are the biggest person-to-person outbreaks of hep A since the late 1990s, when San Diego regularly saw 400 to 600 cases every year—most of them children, many without symptoms. Today’s epidemiology is vastly different. “We have had only one pediatric case, somebody who had not gotten an immunization. All of our other cases are over 25 years old, and the average age is 44,” says Eric McDonald, the medical director for San Diego County Public Health Services’ epidemiology program. The reason: San Diego has more homeless people now, McDonald says.

That made the outbreak harder to fight. In San Diego, teams of public health workers went into the field, trying to convince people to get vaccinated—it’s a two-shot series, so it requires multiple visits. They installed hand-wash stations near homeless encampments, distributed portapotties, and washed streets with bleach solution. “It’s a crisis within a crisis,” says Wilma Wooten, director of public health services for San Diego County. “The homeless situation is a crisis in San Diego, and thrown on top of that is a hepatitis A outbreak.”

It spread—to Santa Cruz, first, and then LA. First to people who’d been in one of the earlier cities, and then to people who had contact with them. And then, in LA, hep A showed up in two more homeless people who had no contacts that traced anywhere else. “And we thought, ‘Uh oh, now there’s local transmission,’” says Jeffrey Gunzenhauser, interim health officer for the Los Angeles County Department of Public Health. It was enough to declare an outbreak.

As in San Diego, LA public health teams spread out to vaccinate as many people as they could. It wasn’t easy. “Here in LA county, with 58,000 homeless and tens of thousands of others using illicit drugs, we were like, whoa, how many of them would we have to vaccinate?” Gunzenhauser says. “When we approach a homeless encampment, for every one of the individuals willing to vaccinate, two or three others are turning it down.”