People living, working or going to school in those areas who also have come into contact with someone who has measles now have three options:

• Get vaccinated.

• Prove that they already were vaccinated.

• Pay a fine of $1,000 for each time they were exposed to the disease.

How will the city know if someone was in contact with a person who has measles?

That’s the job of disease detectives. No, seriously: That’s what they’re called.

Disease detectives figure out who is sick, when they became sick and how they might have gotten sick, according to the federal Centers for Disease Control and Prevention. This information helps doctors and health officials trace the source of an outbreak and ultimately contain it.

Or, if you prefer another reference point: A disease detective is like the character Brad Pitt played in the movie “World War Z.”

Will the city vaccinate people against their will?

No.

“We will not be forcibly vaccinating individuals,” a City Hall spokeswoman, Marcy Miranda, wrote in an email to The Times. Officials “will work with people to educate them about the safety and importance of vaccines and will issue necessary fines as needed,” she wrote.

Why is measles spreading? Wasn’t it eradicated?

Years ago, the number of confirmed measles cases in the United States dropped to a very low point, but the virus was never eliminated.

In 2000, there were no reported cases of the disease in the United States. In 2004, there were 37 confirmed cases in the country. In 2014, that number jumped to 667, according to the C.D.C.