Moviegoers in California may be engaged in a battle of their own, long after watching Avengers: Endgame.

Audience members attending the midnight showing of the movie's premiere may have been exposed to the measles, according to officials.

Federal Health Officials reported more than 700 cases of measles in the United States, and the epidemic is being called the largest outbreak of the preventable disease since 2000, which was the year it was officially declared eliminated.

Now, authorities are issuing a warning in Orange County, California of a known patient who was unaware of having the contagious disease before attending the likely sold-out screening.

The infected moviegoer from Placentia had previously traveled to a country that had experienced an outbreak and contracted the disease, according to the Los Angeles Times.

After returning from abroad and going to work for a few days, she attended one of the first screenings for Endgame on Thursday, April 25, in Fullerton, before realizing she was sick.

Officials did not say whether the woman, who is in her 20s, was vaccinated against the measles.

The warning is one of many during a nationwide breakout with the highest number of cases involving the measles happening in the U.S. within two decades.

Orange County interim health officer Dr. Nichole Quick described symptoms of measles in a statement and suggested those who weren't previously vaccinated should get the MMR vaccine administered.:

"Measles is a highly contagious and potentially severe disease that causes fever, rash, cough, and red, watery eyes. The MMR vaccine is a simple, inexpensive and very effective measure to prevent the spread of this serious virus."

Patients are prone to infecting others within four days after contracting the disease before visible symptoms like rashes, occur.

Officials said those at the following locations should review their vaccination history and monitor themselves for symptoms.

• 5 Hutton Centre Drive, Santa Ana, between 7:45 a.m. and 7:15 p.m. on April 23, 24 and 25.
• AMC movie theater, 1001 S. Lemon St., Fullerton, from 11 p.m. April 25 to 4 a.m. April 26.
• St. Jude Medical Center emergency department, 101 E. Valencia Mesa Drive, Fullerton, from 7 to 9 a.m. April 27.

Anti-vaxx remorse is evident more than ever.

AMC Fullerton employee Carlee Greer-McNeill is rightfully upset over news of everyone that night being exposed.

She told NBC News:

"The last place I would ever expect it was at my job. I wasn't even expecting Orange County. People, if you know you have the measles, please don't come to a movie theater, let alone a public place."

This is the first reported case in Orange County after reports of an outbreak in Los Angeles County last week, in which a resident contracted the measles while traveling to Vietnam earlier this year.

The person infected three other people in L.A. County, leading to 1,000 people being told to stay home for possibly being exposed.