The state of emergency declaration, which lasts for at least 30 days, includes shopping malls, civic centres, schools, restaurants and places of worship in Rockland County, where more than 150 people have been infected since last autumn.
Measles is a vaccine preventable viral illness that has made a resurgence because of the worldwide anti-vaccination conspiracy theories that have prompted many parents to leave their children unvaccinated. "This is a public health crisis, and it is time to sound the alarm, to ensure that everyone takes proper action to protect themselves and their neighbors; for the health and safety of all of us in Rockland", he said in a news release. "The circumstances we're facing here clearly calls for that", he said.
Many vaccines are theoretically mandatory for children to attend school in the United States.
"There will not be law enforcement or deputy sheriffs asking for vaccination records".
Day said only 72.9% of people under 18 have been vaccinated against measles in Rockland County, which has more than 300,000 residents.
Almost half of the 314 measles cases now being reported in the U.S. are coming from Rockland County, according to the Centres for Disease Control and Prevention. People who are medically unable to get vaccinated are exempt.More news: Photos reveal the recent rivers that ran across Mars
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Violating the order will be punishable by a fine of $500 (£378) and up to six months in prison.
The county is experiencing NY state's longest measles outbreak since the disease was declared officially eliminated from the United States in 2000. Health officials say the best way to stop the spread is a vaccination rate of 92 to 95 per cent.
The declaration is an effort to contain an outbreak of measles that began in October; 153 cases have been confirmed in the county.
Almost 17,000 vaccinations have been administered in the county during the outbreak.
The total number of confirmed measles cases in Rockland County has now risen to 156 since the first case was reported last October. The CDC said the outbreaks are linked to travelers who brought the disease back from other countries such as Israel and Ukraine, where large measles outbreaks are occurring.
The CDC has reported 314 cases of measles nationwide this year as of Monday.