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The United States is currently in the midst of the worst measles outbreak since 1994, according to the Centers for Disease Control and Prevention (CDC), and the number of cases continues to rise across the country.

According to a report by the Los Angeles Times, local authorities are warning people that attended a midnight screening of Avengers: Endgame at the AMC Theater in Fullerton, California on April 25 may have been exposed to the disease. A woman in her 20s who attended the screening is the first confirmed case reported in Orange County.

The film had the most successful opening weekend in box office history, so you can be sure it was seen by a lot of people. The Orange County Health Care Agency suggests anyone present at the theater at the time may be at risk of infection and should check their vaccination history and monitor themselves for fever or unexplained rash for up to three weeks.

The measles virus is known to spread through the air and can persist in the environment even after an infectious person leaves the area.

The CDC notes that three counties in California have recorded measles outbreaks since the start of 2019 and 22 US states have reported cases overall. As of writing, 704 cases of measles have been reported in 2019. At the current rate, reported cases in the US are likely to reach their highest number since 1992, when cases topped out at 2,200.

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The agency also notes that the majority of US measles cases are reported after overseas travel. However, outbreaks are also seen to occur in US communities where there are "pockets of unvaccinated people."

"Measles is a highly contagious and potentially severe disease that causes fever, rash, cough, and red, watery eyes," said Nichole Quick, interim county health officer with the Orange County health care agency, in a press release. "The MMR vaccine is a simple, inexpensive, and very effective measure to prevent the spread of this serious virus."

The Orange County Health Care Agency did not immediately respond to a request for comment.