Montana is the ninth U.S. state to have an active measles outbreak.

The U.S. was up to 800 cases of measles nationwide on Friday. Texas is driving the high numbers, with an outbreak centered in West Texas that started nearly three months ago and is up to 597 cases. Two unvaccinated elementary school-aged children died from measles-related illnesses near the epicenter in Texas, and an adult in New Mexico who was not vaccinated died of a measles-related illness.

Other states with active outbreaks — defined as three or more cases — include Indiana, Kansas, Michigan, Oklahoma, Ohio, Pennsylvania and New Mexico. The U.S. has more than double the number of measles cases it saw in all of 2024.

Measles is caused by a highly contagious virus that’s airborne and spreads easily when an infected person breathes, sneezes or coughs. It is preventable through vaccines, and has been considered eliminated from the U.S. since 2000.

Health experts fear the virus will take hold in other U.S. communities with low vaccination rates and that the spread could stretch on for a year.

In North America, an outbreak in Ontario, Canada has sickened 925 from mid-October through April 16. That’s on top of cases in Mexico that the World Health Organization has said are linked to the Texas outbreak. A large outbreak in Chihuahua state has 433 cases as of April 18, according to data from the state health ministry.

Texas state health officials said Friday there were 36 new cases of measles since last Tuesday, bringing the total to 597 across 25 counties — most of them in West Texas. Four more Texans were hospitalized, for a total of 62 throughout the outbreak, and Parmer and Potter counties logger their first cases.

State health officials estimated about 4% of cases — fewer than 30 — are actively infectious.

Sixty-two percent of Texas’ cases are in Gaines County, population 22,892, where the virus started spreading in a close-knit, undervaccinated Mennonite community. The county has logged 371 cases since late January — just over 1% of the county’s residents.

New Mexico announced five new cases this week, bringing the state’s total to 63. Three more people were in the hospital this week, for a total of six since the outbreak started. Most of the state’s cases are in Lea County. Two are in Eddy County and Chaves and Dona Ana counties have one each.

State health officials say the cases are linked to Texas’ outbreak based on genetic testing.

The Ohio Department of Health confirmed 30 measles cases in the state Thursday. The state county includes only Ohio residents.

There are 14 cases in Ashtabula County, 14 in Knox County and one each in Allen and Holmes counties, the state said. The outbreak in Ashtabula County started with an unvaccinated adult who had interacted with someone who had traveled internationally.

Health officials in Knox County, in east-central Ohio, say there are a total of 20 people with measles, but seven of them do not live in Ohio. In 2022, a measles outbreak in central Ohio sickened 85.

Indiana confirmed two more cases Monday in an outbreak that has sickened eight in Allen County in the northeast part of the state — five are unvaccinated minors and three are adults whose vaccination status is unknown. The cases have no known link to other outbreaks, the Allen County Department of Health said Monday.

In far northwest Pennsylvania, Erie County health officials declared a measles outbreak April 14 after finding two new cases linked to a measles case confirmed March 30.

The state has had nine cases overall this year, six of which are not linked to the outbreak, including international travel-related cases in Montgomery County and one in Philadelphia.

Montcalm County, near Grand Rapids in western Michigan, has three linked measles cases. State health officials say the cases are tied to a large measles outbreak in Ontario, Canada.

The state has seven confirmed measles cases as of Thursday, but the remaining four are not part of the Montcalm County outbreak. Michigan’s last measles outbreak was in 2019.

Montana state health officials announced five cases Thursday in unvaccinated children and adults who had traveled out of state, and confirmed it was an outbreak on Monday. All five are isolating at home in Gallatin County in the southwest part of the state.

State health officials are working to trace exposures in Bozeman and Belgrade.

They are Montana’s first measles cases in 35 years. Health officials didn’t say whether the cases are linked to other outbreaks in North America.

There have been 800 cases in 2025 as of Friday, according to the U.S. Centers for Disease Control and Prevention, and 10 clusters — defined as three or more related cases.