Public health officials in Los Angeles County have declared an outbreak of hepatitis A, a highly contagious liver infection driven by a virus that can, in rare cases, cause severe illness.

The condition, which is typically identified in fewer than 50 people in L.A. County each year, infected at least 138 people in 2024 and cases have remained unusually high so far in 2025. Officials say that levels of the virus in local wastewater suggest these figures are an undercount.

Here’s what to know.

How is hepatitis A transmitted?

The hepatitis A virus is spread through the so-called oral-fecal route, which means it is ingested through food or drinks that have been contaminated by microscopic bits of stool from an infected person. It can also spread through close contact with someone who is carrying the virus, even if they don’t show any symptoms.

Unhoused people are often at greater risk of contracting hepatitis A because they have limited access to proper toilets and hand-washing facilities. The virus is also more common among travelers to places with poor sanitation, men who have sex with men and people who use drugs.

But officials say many of the recent cases have occurred in people without those risk factors. They do not yet know why, but Dr. Timothy Brewer, an infectious disease physician and epidemiologist at U.C.L.A., said it had led him to worry about two things: food contamination, and community circulation.

“If you’re practicing good hygiene in a community that has access to potable water, you really should not be seeing a lot of this,” he said. “Something is going on — but exactly what that something is, we don’t yet know.”