Upward of 500 people might have been exposed to the measles after someone with the virus entered a Gresham child care center, Multnomah County health officials said Monday.

The measles infection was confirmed Wednesday after an emergency room visit. County health officials, Adventist Health Portland and the child care center have since issued notifications to anyone who might have come into contact with the patient.

There are about 40 people known to have been exposed who've not been immunized, county health officials said Monday.

More than 90 percent of the county's adolescents have been vaccinated against measles. Those most at risk of contracting the illness include people who have not been inoculated, pregnant women, babies or those with weakened immune systems, according to health officials.

The virus can spread through the air when an infected person coughs or sneezes. It can linger in the air for hours sometimes.

A person who's been infected typically will show symptoms within two weeks, though sometimes it takes longer. Multnomah County health officials estimate that anyone infected in the Gresham case would show symptoms by mid-July.

Symptoms could manifest as a fever, cough, runny nose, red eyes or red rash that usually begins on the head and spreads to the rest of the body. If untreated, measles can lead to ear infections, lung infections and diarrhea.

Occasionally, dangerous swelling of the brain occurs.

Measles remains rare in the United States, said Dr. Jennifer Vines, a Multnomah County deputy health officer. She said that most U.S. residents pick up the virus outside the country and bring back with them.

Two doses of the vaccine are required to be fully inoculated. Even then, you could still be at risk if you live in an area with low vaccination rates.

Measles used to infect about 4 million people a year in the United States, hospitalizing 50,000 and killing 500. But widespread vaccination starting in the 1960s led the federal Centers for Disease Control and Prevention to declare the disease eradicated in 2000.

It's made a comeback with the rise in skepticism about vaccines, and as U.S. residents travel to countries where the disease is still common. In 2015, nearly 150 people contracted measles in seven states, including Oregon. One woman in Washington state died.

Oregon charter schools in some areas are below the threshold for measles group immunity, according to an Oregonian/Oregonlive analysis earlier this year.

"Because measles is so contagious, high levels of immunity in the community are needed to prevent its spread," Vines said. "Now is a very good time to make sure you and your children have been vaccinated against measles, especially if you are planning to travel this summer."

Contact a primary care provider or local health department if you have questions about measles and vaccinations.

If you think you are infected, call a doctor or emergency room first before visiting to avoid exposing people in the waiting room to the virus.

-- Molly Harbarger

mharbarger@oregonian.com

503-294-5923

@MollyHarbarger