US measles cases rising in 2024. But what’s causing the increase?

FILE-A vaccine for measles is prepared by a health professional (Photo by: BSIP/Universal Images Group via Getty Images)

Measles cases are climbing in the U.S. this year, raising concerns for health officials. 

In 2024, measles cases are already nearly double the total for last year. 

The CDC released a report Thursday on recent measles case trends, explaining that cases in the first three months of 2024 were 17 times higher than the average number seen in the first three months of the previous three years.

RELATED: CDC updates measles prevention guidelines for travelers

Measles is one of the world’s most contagious and infectious diseases that may cause serious complications, with experts sharing that getting vaccinated is the best defense against measles. 

How many measles cases has the U.S. seen in 2024?

The Centers for Disease Control and Prevention recorded 113 measles cases as of April 5. There have been seven outbreaks and most U.S. cases — 73% — are linked to the outbreaks.

According to the Associated Press, the count is lower than in some recent years: 2014 saw 667 cases and 2019 had 1,274.

Where were this year's U.S. measles outbreaks?

There are measles cases in 17 states to date in 2024, including in New York City, Philadelphia, and Chicago.

According to the AP, roughly half of this year's cases come from the Chicago outbreak, where 61 people have contracted the virus as of Thursday, primarily from people living in a migrant shelter.

RELATED: CDC dispatches team to Chicago as measles cases prompt concerns

The city health department told the AP that cases are down after health officials administered 14,000 vaccines in a month.

How does measles spread?

Measles is transmitted by direct contact with infectious droplets or by an airborne spread when an infected person breathes, coughs, or sneezes.

According to the Centers for Disease Control and Prevention, measles can remain infectious in the air for up to two hours. About 9 out of 10 vulnerable people will get the virus if they are exposed.

Where is measles coming from?

The disease is still common in many parts of the world, and measles reaches the U.S. through unvaccinated travelers.

Citing a travel report from Thursday, the AP noted that many of the recent cases involved unvaccinated Americans who got infected in the Middle East and Africa and brought measles back to the U.S.

RELATED: If you were vaccinated for measles in the 1970s and 80s you may not be protected: Doctors

The CDC says people at high risk for severe illness and complications from measles include infants and children younger than 5, adults older than 20 years old, pregnant women, and individuals with compromised immune systems, including from leukemia and HIV infection.

The Associated Press contributed to this report. This story was reported from Washington, D.C.