According to the Orange County Health Care agency, a 20-something woman, who did not know at the time that she had measles, was in the audience for the April 25 show. She is considered infectious between April 23 and through Wednesday.
She is believed to be in her 20s and to have entered the theater at 11 p.m. and didn't leave until 4 a.m.
The situation occurred over the weekend at an AMC movie theater in Orange County, California.
The biggest concern has been local residents traveling to states like NY or New Jersey, where there have been outbreaks, Allegheny County Health Department officials said during a Board of Health meeting.
The woman, who had just returned from an global trip, didn't know she had the very contagious virus.
Measles cases in the US are at the highest on record "since the disease was declared eliminated nationwide in 2000", CNN reported.
The woman has now voluntarily quarantined herself at her home in Orange County while officials try to identify anyone who might have been exposed at the AMC movie theater in Fullerton.
The warning comes amid several major measles outbreaks nationwide and the highest number of measles cases across the nation in more than two decades.
The CDC reports that 22 states have experienced more than 700 cases of measles in 2019.
An Allegheny County resident - an unvaccinated adult who recently traveled overseas - was treated for measles Monday in UPMC Shadyside's emergency room and is recovering at home.
Health officials are urging people to vaccinate if they have not done so already.
The disease is highly contagious and spreads through the air when an infected person coughs or sneezes.
Officials are also reminding people who think they may have the measles to call their doctor before going to the physician's office to prevent infecting others.