A traveler infected with measles may have spread the highly contagious disease while riding an Amtrak train from Chicago to Los Angeles and then taking trains and buses across at least six California counties and a train to Denver.

The unidentified person, who wasn’t vaccinated against measles, was hospitalized after the five-day trip from September 10 to 15, according to the California Department of Public Health, which on Friday urged Golden State residents to protect themselves by getting vaccinated.

“This case, along with previous measles outbreaks in California this year, serve as reminders of the importance of vaccination against measles,” Dr. Rita Nguyen, the state’s assistant health officer, said in a statement. “Measles is one of the most contagious infections that can lead to severe life-long consequences including permanent brain damage and can also be fatal, especially for children.”

The CDPH posted the traveler’s itinerary online and said anyone who was on the trains or buses, or was at the stations on the relevant days, may have been exposed to measles.

It also noted that the traveler’s scheduled arrival and departure times may differ from the actual times, and said state health officials were working with federal and county officials to notify the people who may be at risk.

The disturbing development came as the U.S. is grappling with a record measles caseload that’s the highest since 1991, according to the University of Minnesota’s Center for Infectious Disease Research & Policy.

The nation has experienced multiple measles outbreaks in 45 states and the District of Columbia this year, with the Centers for Disease Control and Prevention reporting 3,471 confirmed cases as of Friday.

Only five states — Arkansas, Hawaii, Mississippi, Nevada, New Hampshire — haven’t reported any infections, according to the CDC.

Ninety-five percent of the confirmed cases involve people who haven’t been vaccinated or whose vaccination status is unknown.

Pennsylvania has recorded four measles related deaths, but they’re not listed on the CDC’s website, which says that the National Center for Health Statistics “does not currently have any death records from 2026 that indicate measles is the underlying cause.”

President Donald Trump’s health secretary, former environmental lawyer and vaccine critic Robert F. Kennedy Jr., has repeatedly downplayed America’s rash of measles cases by blaming them on a global epidemic and claiming the U.S. is handling it better than other countries.

Measles data compiled by the World Health Organization show that the U.S. had 13.2 measles cases per 1 million people this year, as of September 14, FactCheck.org reported Friday.

That rate makes America No. 65 for the highest rate of infection among 169 countries with measles data, or among the worst 38 percent, according to FactCheck.org.

“There are countries that are sort of peer countries — you know, peer developed countries — which have very, very little, basically no outbreaks of measles,” Josh Michaud, an associate director of global and public health policy at the health policy research organization KFF, told the website.

A spokesperson for the Department of Health and Human Services didn’t immediately respond to a Saturday inquiry from The Independent.