On Tuesday, New York City officials declared a public health crisis after finding 285 people in a Brooklyn neighborhood were infected with measles.

Upon inspection of the outbreak, they found large groups of unvaccinated children and communities with extensive anti-vaccine campaigns.

During a press conference, officials also said they received reports of New York City parents hosting "measles parties" to infect their unvaccinated children with the disease and help them get it over with.

Doing so can result in serious illness and even death.

Visit INSIDER's homepage for more.

On Tuesday, New York City officials declared a public health crisis after confirming 285 cases of measles in the Brooklyn-based neighborhood of Williamsburg.

Most of the people who contracted the disease appear to have come from predominantly Orthodox Jewish communities, where large groups of children remain unvaccinated. Now, Mayor Bill de Blasio is requiring vaccinations in certain zip codes in an effort to contain the outbreak, and anyone who doesn't get the vaccine within 48 hours could be fined up to $1,000.

But news of "measles parties," events where parents bring their unvaccinated children together to purposely help them contract the disease, are concerning public health officials. During a press conference on the measles outbreak, officials confirmed multiple reports of these gatherings, which echo other disease-related "parties" of years past.

"Back in the day people were having parties to expose their kids to chickenpox," NYC Health Commissioner Dr. Oxiris Barbot said. "We live in a different world now ... there are serious consequences to that."

Barbot is right. In March, Kentucky Governor Matt Bevin said he purposefully exposed his nine unvaccinated kids to chickenpox, and he isn't alone. The chickenpox vaccine became available in 1995, but before then, these gatherings were fairly popular and considered a way to help kids "get it over with."

These parties operate on the premise that most people who contract chickenpox once never get it again, but that doesn't make them safe.

Read more: Measles is a public health emergency in the US — these 15 charts explain the vaccine problem

Whether it's with chickenpox, measles, or another infectious disease, such gatherings can have serious health implications since it's impossible to predict how severe a person's reaction to the disease will be.

"Some kids will just get a few [chickenpox], some will die. You just don't know, so we vaccinate everyone [we can]," pediatrician Dr. Natasha Burgert previously told INSIDER. She also called the practice of infectious disease parties a "gamble."

The best way to prevent infectious diseases including measles is with a vaccine. Currently, the measles or MMR vaccine protects against the mumps, measles, and rubella and the Centers for Disease Control and Prevention (CDC) recommends children get their first dose of the vaccine when they are between 12 and 15 months old. The vaccine is administered in two doses, and the second one is recommended when a child is between 4 and 6 years old.

According to the CDC, two doses of the vaccine are around 97% effective at preventing measles, while one dose is around 93% effective. There is also a vaccine available to protect against the three aforementioned diseases as well as chickenpox.

"Widespread use of measles vaccine has led to a greater than 99% reduction in measles cases compared with the pre-vaccine era," the CDC wrote on its website.