The U.S. has 800 cases of measles nationwide as of Friday, and two more states identified outbreaks this week.

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.

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

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

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.

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.

Getting another MMR shot is harmless if there are concerns about waning immunity, the federal Centers for Disease Control and Prevention says.

People who have documentation of receiving a live measles vaccine in the 1960s don’t need to be revaccinated, but people who were immunized before 1968 with an ineffective measles vaccine made from “killed” virus should be revaccinated with at least one dose, the agency said.

People who have documentation that they had measles are immune and those born before 1957 generally don’t need the shots because most children back then had measles and now have “presumptive immunity.”

In communities with high vaccination rates — above 95% — diseases like measles have 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. The U.S. saw a rise in measles cases in 2024, including an outbreak in Chicago that sickened more than 60.

Measles first infects the respiratory tract, then spreads throughout the body, causing a high fever, runny nose, cough, red, watery eyes and a rash.

Most kids will recover from measles, but infection can lead to dangerous complications such as pneumonia, blindness, brain swelling and death.