With nearly 20 measles cases so far this year, Colorado is on track to record its highest number of infections since the disease was declared eradicated in the U.S. in 2000.

Last week, the Colorado Department of Public Health and Environment confirmed a measles case in an unvaccinated Lake County teenager.

As of April 28, the state has had 17 cases in six counties, according to state data.

To put that number into perspective, Colorado only had six measles cases between 2014 and 2024.

At this point last year, Colorado had three cases.

“Measles activity in Colorado mirrors what has been occurring throughout the U.S. over the past two years,” Hope Shuler, a health department spokesperson, said in an email to The Denver Gazette.

Health officials began sounding the alarm earlier this year when outbreaks across the United States dramatically increased the number of measles cases.

The U.S. recorded 2,242 measles cases in all of last year. As of last week, the country had already reached 1,792 cases — putting 2026 on pace to approach or exceed last year’s total with more than half the year still to go.

Childhood immunizations have fallen nationally, and Colorado’s kindergartners are no exception.

Over the past five years, the state’s vaccination rate dropped among kindergartners from 91.1% in the 2019-2020 school year to 88% last school year.

To be protective in a community, 95% or more of students should be vaccinated against measles.

“These declining rates directly increase the risk of outbreaks of measles,” Shuler said.

Measles was declared eradicated in 2000, thanks in large part to a highly effective vaccination campaign.

A highly contagious and preventative respiratory illness, measles is transmitted through direct contact with infected droplets or when an individual breathes, coughs or sneezes.

Symptoms typically emerge within two weeks after exposure.

The telltale sign of a measles infection is a spotty red rash. In rare cases, measles can cause swelling of the brain and even death.

Before the vaccine, an estimated 48,000 Americans were hospitalized with measles and 500 died each year.

Since 2014, Colorado has reported zero measles cases in six of 11 years, with only one or two cases in most of the others. This changed in 2024, when the state recorded 36 cases.

That’s six times as many cases than the previous decade — with roughly one in four associated with an out-of-state traveler who flew while infectious, according to the state.