Chicago resident who traveled through O'Hare Airport diagnosed with measles

As measles cases are on the rise in the United States, Chicago has reported the city's first case of the year.

The Chicago resident was an adult who traveled internationally through O'Hare Airport in early April, according to public health officials.

The individual -- who had one prior dose of the measles vaccine -- had a rash onset on April 25 and has been isolated at home since being diagnosed, officials said.

Breakthrough infections, when someone still gets sick despite being vaccinated, are rare but possible, according to the CDC.

Vaccinated people who get measles are less likely to spread the disease to other people and typically have milder illness, the CDC notes.

Experts are not sure why breakthrough infections occur -- it is possible the immune system did not respond as well as they should have to the vaccine.

The Chicago Department of Public Health and Cook County Department of Public Health released a press release regarding the case, urging residents to receive two doses of the vaccine at least two weeks before travel.

Separately, a second measles case that was reported in Illinois on Wednesday was an adult who is a suburban Cook County resident. They visited a local hospital for medical care on Monday and were isolated.

Their vaccination status is unknown, public health officials said.

Chicago and Cook County health officials have worked to notify anyone who may have been exposed to the sick individuals are urging the public to ensure they are up to date on their vaccinations.

Individuals who may have been exposed and are unsure about their vaccination status should notify their health care provider for further instructions, according to health officials.