(NEXSTAR) – Measles, one of the most contagious viruses in the world, has been confirmed in 17 states and counting as outbreaks multiply around the country.

As of last week, the Centers for Disease Control and Prevention said at least 378 cases of the virus have been confirmed in 2025 – and that number is expected to keep rising.

“What is interesting about this current outbreak is the speed at which it’s expanding and increasing,” said Dr. William Moss, an epidemiology professor at the Johns Hopkins Bloomberg School of Public Health and director of the school’s International Vaccine Access Center.

In a briefing this month, Moss said he expects measles will continue spreading in the U.S. as long as we still have these two conditions in place: a susceptible community and an infected person.

“I like to think of this like a forest fire with sparks spreading out. If those sparks land in a community with low vaccine coverage, we’re going to see a larger measles outbreak. If it lands in a community with high vaccine coverage, we may only see one or a few cases,” he said.

Three areas of the U.S. (so far) are experiencing what the CDC characterizes as a measles outbreak, which are three or more cases related to each other. The largest is in West Texas, followed by New Mexico and Kansas.

There are more than a dozen other states reporting cases, but the spread is happening fastest in areas where many are unvaccinated against the virus.

In communities with high vaccination rates — above 95% — measles is having a harder time spreading through communities. This is called “herd immunity.”

But childhood vaccination rates have declined nationwide since the pandemic and more parents are claiming religious or personal conscience waivers to exempt their kids from required shots.

“I like to say vaccines are really a victim of their own success,” said Dr. Anna Durbin, a vaccine researcher at Johns Hopkins. Parents may think they can skip the measles vaccine because they don’t believe it’s a serious threat in modern times, and haven’t heard of many children getting seriously sick or dying.

But this logic doesn’t hold up, Durbin said. “The reason parents aren’t familiar with these diseases is because vaccines have been so effective in controlling them and preventing children from becoming ill and dying.”

Two unvaccinated people this year have died from measles-related causes.

The best way to avoid measles is to get the measles, mumps and rubella (MMR) vaccine. The first shot is recommended for children between 12 and 15 months old and the second between 4 and 6 years old.

People at high risk for infection who got the shots many years ago may want to consider getting a booster if they live in an area with an outbreak, said Scott Weaver with the Global Virus Network, an international coalition. Those may include family members living with someone who has measles or those especially vulnerable to respiratory diseases because of underlying medical conditions.

The Associated Press contributed to this report.