Measles virus Cynthia S. Goldsmith

New York health officials are investigating an outbreak of measles in the city after 16 people tested positive in two of the city’s five boroughs, reports CBS.

Officials do not yet know where or how the outbreak began, but they are urging everyone who is not vaccinated to get the vaccination as soon as possible. Measles infects 90 percent of unvaccinated people who are exposed to it.

Four children too young to be vaccinated, three vaccinated children just over the age of 1, two unvaccinated children, and seven adults ranging in age from 22 to 63 have been infected in Manhattan and the Bronx.

Measles was virtually eliminated in the United States in 2000, but it still exists in other parts of the world. There were 122,000 measles deaths in 2012, mostly children under the age of 5, according to the World Health Organization.

But measles can still find its way to the U.S. There were 175 cases in 2013, almost all of which were linked to people traveling abroad. Fifty-four people have been infected in 2014, around a dozen of which had recently traveled to the Philippines.

Some speculate the spike also has to do with the anti-vaccination sentiment gaining steam in the U.S. Some anti-vaccine proponents say there are links between the vaccine and other conditions like asthma and autism, and instead opt for more “natural” solutions, like exposing their children to viruses to allow their immune systems to tackle it on its own.

Health officials warn against this, saying that unvaccinated children are at risk and also pose a risk to others.

The measles vaccine was first developed in 1963 and compounded into the MMR (measles-mumps-rubella) vaccine in 1971. It is considered 95 percent effective and considered safe by the World Health Organization.