Eight measles cases in Northern California have been classified as an outbreak of the highly contagious disease, state health officials said.

The outbreak is part of a national surge in measles cases that has the California Department of Public Health “strongly urging” Californians to be sure their measles vaccinations are up to date, the agency said in a news release Monday.

A total of 17 measles cases have been reported statewide, including cases in Los Angeles, Orange, San Bernardino, Riverside and Shasta counties.

Two people with measles visited Disneyland on Jan. 22 and Jan. 28 while infectious, health officials said.

In Shasta County, the eight measles cases are all related, officials said. All patients were either unvaccinated or had an unknown history of vaccination.

“The United States is experiencing the highest numbers of measles cases, outbreaks, hospitalizations and deaths in more than 30 years, driven by populations with low vaccination rates,” said Dr. Erica Pan, department director and state public health officer.

South Carolina has had one of the largest outbreaks in 30 years, with more than 920 associated measles cases, California health officials said.

Here’s what to know about measles and what constitutes an outbreak:

What is an outbreak?

The Centers for Disease Control and Prevention defines an outbreak as taking place when more cases than expected are reported, or when cases are clustered by time, space or common behaviors.

The World Health Organization similarly defines an outbreak as a sudden, unexpected increase in cases of a specific disease or health condition within a defined community, area, or season.

An epidemic is larger and more widespread than an outbreak, and a pandemic spreads over multiple nations or continents.

What is measles? Where are outbreaks in the U.S.?

Measles is a “highly contagious” virus that spreads through the air when an infected person coughs or sneezes, according to the CDC.

The disease can cause “serious health complications,” especially in children younger than 5 years old, the federal public health agency said.

Complications due to measles can include pneumonia, “brain swelling, even blindness and death,” Chin-Hong said.

Measles was declared eliminated in the United States in 2000, the CDC said, meaning that the continuous spread of the disease was stopped for longer than 12 months.

Already, as of Feb. 5, 20 states have reported measles cases.

Why are measles cases increasing?

About 90% of unvaccinated people who come in contact with measles will get sick with the disease, the California Department of Public Health said.

The highly contagious virus can live airborne in a room for up to two hours, the agency said, and it only requires a small amount of particles to infect someone.

Measles rates are primarily rising due to high rates of the disease in other parts of the world paired with falling vaccination rates in the United States, Chin-Hong said.

Cuts to public health funding have also contributed to the spread, he added, along with a wave of fake conspiracy theories that mislead people about the importance of getting vaccinated.

Chin-Hong compared the country to a bone-dry forest that’s vulnerable to wildfires.

“You just light a match and the whole thing goes ablaze,” he said. “That’s what happened in West Texas and New Mexico, and a little bit in Oklahoma.”

Disease expert predicts more cases, hospitalizations and deaths

Many public health officials suspected the United States will lose its measles elimination status by the end of 2025, according to Chin-Hong. It hasn’t yet, but 2026’s outbreaks may do so.

That would mean there’s sustained measles transmission in local communities for more than 12 months, he said, without the disease being introduced from outside the country.

Chin-Hong predicted that there would be “more cases ... more hospitalizations, more complications, more deaths potentially,” if the United States loses its elimination status.

It would also be a “psychological blow,” he said. “It’s embarrassing because we have the resources to deal with it.”

What are common measles symptoms?

Measles cases typically start with a high fever, cough, runny nose and watery eyes, according to the CDC.

After a few days of symptoms, tiny white spots pop up inside the mouth, the public health agency said, followed by a rash of small red spots that appear on the face and then the rest of the body.

Symptoms typically appear about seven to 14 days after being exposed to the virus, the national public health agency said.

Here are some of the most common measles symptoms, according to the CDC:

—High fever

—Cough

—Runny nose

—Red, watery eyes

—Rash

Complications from measles usually include ear infections and diarrhea, the CDC said, while more serious complications include pneumonia and encephalitis.

“Some people think of measles as just a little rash and fever that clears up in a few days,” the CDC warned. “But measles can cause serious health complications.”

Who is most at risk of measles?

“Anyone who is not protected against measles is at risk” of getting sick, according to the CDC.

What’s the best way to protect against the virus?

“The best way to protect against measles is with the measles, mumps and rubella vaccine,” the CDC said.

The federal health agency said the two-dose vaccine is safe and about 97% effective at preventing measles.

“The vaccine is so amazing, it’s probably one of the best vaccines we have ever had,” Chin-Hong said.

Children can get vaccinated against measles as early as 6 months old.

The CDC recommends that kids get one dose between ages 12 to 15 months of age, and the second dose at 4 to 6 years old.

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