Measles is a one of the world's most contagious viruses. What starts as a fever can cause serious health complications and even death.

The U.S. eliminated the disease in 2000.

But an ongoing outbreak in New York could change the country's 'measles-free' status.

Millions of Americans suffered from measles in the decades before the U.S. eliminated the disease in 2000. But if an ongoing outbreak in New York state continues into next month, the U.S. could lose its "measles-free" status.

"As a global leader in public health, it is mortifying that (the U.S.) may lose its measles elimination status," said Dr. Paul Spiegel, director of the Johns Hopkins Center for Humanitarian Health. "The measles outbreaks should not have occurred as vaccination against measles is very effective."

The U.S. isn't alone. Four countries — Albania, Greece, the Czech Republic and the United Kingdom — recently lost their measles elimination status, which the World Health Organization defines as the absence of continuous disease transmission for greater than 12 months.

The U.S. would immediately lose its elimination status if there is any case connected to the current outbreak on or after Oct. 2, according to CDC spokeswoman Kristen Nordlund. If there's a case that was not transmitted through the current outbreak, however, then the U.S. would not lose elimination status.

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Reported cases worldwide rose by 300 percent in the first three months of 2019, compared to the same period in 2018, according to preliminary data from the WHO. And that's likely an underestimate: WHO estimates that less than 1 in 10 cases are reported globally.

“Re-establishment of measles transmission is concerning. If high immunization coverage is not achieved and sustained in every community, both children and adults will suffer unnecessarily and some will tragically die,” said Dr. Günter Pfaff, chair of the European Regional Verification Commission for Measles and Rubella Elimination, in a press release last month.

Before the introduction of a vaccine in 1963, measles epidemics occurred about every two to three years worldwide and caused an estimated 2.6 million deaths each year, according to the WHO. In 2017, about 110,000 people died from measles, mostly children under the age of 5 years.

Three outbreaks ongoing in the US

This year, the U.S. has seen its highest number of reported cases since 1992. There were 1,241 individual measles cases confirmed across 31 states from Jan. 1 to Sept. 5, according to CDC.

Only two states — New York and Texas — are currently experiencing outbreaks (defined as three or more cases). The nation's largest outbreak this year, concentrated in Orthodox Jewish neighborhoods of New York City, ended earlier this month.

But three others are ongoing: in El Paso, Texas, and in New York's Rockland and Wyoming counties.

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More than 75% of the cases reported this year are linked to outbreaks in New York. They can be traced back to travelers who brought measles from other countries such as Israel, Ukraine and the Philippines, where large measles outbreaks are occurring, CDC says.

The ongoing outbreak in Rockland County began when an international traveler who had measles flew through Newark Liberty International Airport on Sept. 28, 2018, potentially exposing others to measles from Sept. 28 to Oct. 1, according to local authorities. Reports of rash onset in the county began on Oct. 1, 2018.

New York Health Commissioner Dr. Howard Zucker said Friday that his department was working closely with community stakeholders and local health departments to stop any continued transmission of measles in the state.

"Since the outbreak began last October, more than 71,000 doses of MMR vaccines have been administered in the counties where we’ve sustained our most aggressive public health response, a 70% increase in the number of vaccines administered from the previous year," Zucker said.

Measles is a one of the world's most contagious viruses. What starts as a mere fever can cause serious health complications and even death. CDC estimates that about 1 in 5 people in the U.S. who get measles will be hospitalized, and 1 to 3 out of 1,000 will die.

Why is measles back?

The global resurgence has been fueled in part by the anti-vaccination movement. While the measles vaccine is 97% effective, most cases in the U.S. this year have occurred among people who were not vaccinated, CDC says.

Proponents of the movement push the discredited theory that the MMR vaccine, administered to children, causes autism. Celebrity anti-vaxxers and social media campaigns have continued to champion this viral theory.

“Probably the No. 1 factor we have to fight is misinformation, this concept some people have that vaccines are dangerous," Judd Hultquist, assistant professor of infectious diseases at Northwestern University Feinberg School of Medicine, told USA TODAY earlier this year.

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This year, New York City bore the brunt of that misinformation campaign, which targeted Orthodox Jewish neighborhoods. In April, Mayor Bill de Blasio declared a public health emergency and required unvaccinated people living in designated ZIP codes to receive the vaccine.

The city's health commissioner Oxiris Barbot said at the time that the outbreak was "driven by a small group of anti-vaxxers" in the targeted neighborhoods.

"They have been spreading dangerous misinformation based on fake science,” she said. “We stand with the majority of people in this community who have worked hard to protect their children and those at risk."

The best way forward is to launch a "robust response to the anti-vaccine empire," said Dr. Peter Hotez, professor of Pediatrics and Dean for the National School of Tropical Medicine at Baylor College.

“We need to take aggressive steps to restore vaccine confidence and vaccine acceptance," Hotez said.

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How can the US regain elimination status?

To requalify for elimination status, the U.S. would need to demonstrate that there was no continuous measles transmission in the country for at least a year.

Nordlund said the CDC plans to publish a webpage on elimination status sometime next week.