The United States so far this year has recorded 839 cases of measles in 23 states, its largest outbreak since public health officials in 2000 declared the disease had been eradicated, the Centers for Disease Control and Prevention said Monday.

Following are some facts about the outbreak, based on data from the CDC; the World Health Organization; public health offices in New York, California and Michigan; and the National Conference of State Legislatures:

Public health officials blame the measles resurgence on the spread of misinformation about vaccines. A vocal fringe of parents opposes vaccines, believing, contrary to scientific studies, that they can cause autism.

The largest outbreaks are concentrated in New York City, which has recorded at least 466 cases, primarily in the Orthodox Jewish community in the Williamsburg neighborhood of Brooklyn, and also in suburban Rockland County north of New York City, recording at least 224 cases. Those figures include infections from last year and are not directly comparable to the CDC numbers.

Other outbreaks are ongoing in California, Michigan, New Jersey, Georgia and Maryland.

The disease is highly contagious and can be fatal, killing one or two of every 1,000 children who contract it, according to the CDC. It can also cause permanent hearing loss or intellectual disabilities. It poses the greatest risk to unvaccinated young children.

The United States’ 2000 declaration that measles had been eradicated meant that the disease was no longer present in the country year-round. The disease remains common in some countries in Europe, Asia and Africa, and unvaccinated travelers to those countries can bring it back to the United States. The current outbreaks are believed to trace back to visits to Israel and Ukraine.

New York City officials said 22,833 people have received the measles-mumps-rubella (MMR) vaccine in affected areas of the city since the outbreak began in October. The city has begun fining unvaccinated adults.

Lawmakers in Oregon, New Jersey, New York, Iowa, Vermont and Minnesota are considering bills to eliminate non-medical exemptions that allow unvaccinated children to attend public schools. Only three states currently bar all non-medical exemptions: California, Mississippi and West Virginia.

To achieve herd immunity that protects those unable to get the measles vaccine, such as infants and people with compromised immune systems, 90 to 95% of the population needs to be vaccinated, health officials say.

A graphic on the outbreak can be viewed at tmsnrt.rs/2GJgoBt.