Health officials in Washington’s Clark County, which sits north of Portland, Ore., said that 17 of the cases in the outbreak involved children under the age of 10, with another four between 11 and 18 years old. At least 19 of the patients were not immunized against measles.

Officials told CNN that one infected patient attended a Portland Trail Blazers home game last week, while others traveled to airports, schools, churches, and restaurants.

The Clark County measles outbreak comes only four months after New York State recorded 167 cases of measles in September, in what was one of the worst outbreaks in the U.S. in decades. An outbreak is generally defined as three or more linked cases of measles infections.

The first three weeks of 2019 has already seen 22 infections in Washington, or one new case per day. At that rate, this year would surpass 2018 in terms of measles infections. The reasons for the rise in new measles cases are related to increased travels to areas where measles is more common, as well as the vulnerability of communities in the U.S. where people choose not to vaccinate their families against the disease.

“In a given year, more measles cases can occur for any of the following reasons: an increase in the number of travelers who get measles abroad and bring it into the U.S., and/or further spread of measles in U.S. communities with pockets of unvaccinated people,” the Centers for Disease Control said.

According to the CDC, 2018 was the second-worst year for measles cases since 2000. Last year saw 17 outbreaks that led to 349 infections. Many of them occurred in New York and New Jersey after unvaccinated people returned home from trips to Israel, where a large outbreak was occurring.

“Eighty-one people brought measles to the U.S. from other countries in 2018,” a CDC factsheet said. “This is the greatest number of imported cases since measles was eliminated from the U.S. in 2000.”