Measles is now riding trains and going to ballgames — the one travel check that matters takes five minutes at your doctor's office

An unvaccinated passenger with measles rode trains and buses across the country and through several California counties, and the state health department had to notify everyone who might have shared the air. In St. Louis, the city health department asked residents to watch for symptoms after someone who had visited local bars and a sports stadium tested positive. NPR rounded up both alerts this week under a question a lot of people are now asking: should I still travel?

For most readers, the answer is yes — if you know your vaccine status. That is the part many adults cannot actually answer, and it is the part that is cheap and fast to fix before you pack.

Why a train car or a stadium is a measles problem

Measles is not passed by a handshake. It travels through the air when an infected person coughs, sneezes or simply breathes, and you can catch it by walking into a space up to two hours after they left. A packed concourse, a boarding gate, a long-distance train — all shared airspace.

The timeline makes it worse. People are contagious for about four days before the telltale rash appears and four days after, according to Dr. Kalyani McCullough of the California Department of Public Health. Symptoms usually start 7 to 14 days after exposure, sometimes as late as 21. So the person sitting near you on day one has no idea they are sick, and neither do you until weeks later.

That is why epidemiologists spend days reconstructing an infected person's movements. "It's a really labor-intensive process," McCullough told NPR. Exposure alerts are the output — and they are arriving more often.

The numbers are probably low

The CDC lists more than 3,600 confirmed cases this year, with Pennsylvania, Ohio and New York leading over the past two weeks. Dr. Paul Offit of Children's Hospital of Philadelphia thinks the real figure is higher, because many families never seek care for what looks like a fever and a rash. "If anything, we're underestimating," he told NPR.

Even the death toll is contested. The CDC counted two deaths as of Sept. 28; Pennsylvania's health department has counted four measles-associated deaths, and the CDC has disputed whether measles caused them. Whichever count you trust, the disease is not "just a rash" — McCullough points to "immune amnesia," in which the virus wipes out some of the body's existing protection against other infections.

Who should actually worry

Two doses of the MMR vaccine (measles, mumps and rubella) are considered protective, and experts told NPR that fully vaccinated, generally healthy travelers do not need to change their plans. "I'm not worried about those individuals," said George Mason University epidemiologist Amira Roess.

The people with real exposure risk are:

• The unvaccinated — up to nine in ten exposed will get infected.
• Babies under 12 months, who normally get their first dose around their first birthday.
• People who are immunocompromised and cannot get a live vaccine, who depend on everyone around them being immune.
• Adults who are not sure — the large, quiet group who think they were vaccinated as kids but have no record.

That last group matters because protection only works community-wide when about 95% are immune. Kindergarten coverage dipped to 92.4% last school year. The gap is where outbreaks live.

What to do before your next trip

• Find your record. Your state immunization registry, an old pediatrician, or a school or college health office may have it. Our guide on getting your medical records explains your right to a copy.
• No record? Call your primary care office. Your clinician can tell you whether to get a dose or check immunity with a blood test — that is their call, not ours. If you do not have a regular doctor, start with our guide to finding a primary care doctor.
• Traveling abroad with a baby 6 to 11 months old? The CDC recommends an early MMR dose before international travel. Ask the pediatrician now, not the week you fly.
• Check the cost before the shot. Routine recommended vaccines are generally covered without a copay by most private plans when given in network, as part of the ACA's preventive-care rules — see our guide to free preventive care.
• If you get an exposure alert, call your doctor or local health department before showing up anywhere, and isolate if symptoms appear within three weeks. Walking into a crowded waiting room with a fever and rash is how one case becomes ten.

Masks in crowded spaces are a reasonable extra layer if you are concerned, Roess said. But no airline seat, train ticket or stadium gate decides your risk here. A two-line entry in a vaccine record does — and most of us have never looked for it.