If you take the long view, international health organizations have much to be encouraged about when it comes to the global fight against measles. From 2000 to 2017, for instance, the annual number of measles-related deaths dropped 80 percent — from a toll of over half a million to just under 110,000 last year.

But lurking inside those statistics, published Thursday by the World Health Organization and the Centers for Disease Control and Prevention, are some far less rosy numbers. Specifically, what's been going on recently.

The report found that cases of the highly contagious disease spiked by more than 30 percent from 2016 to 2017. The WHO and CDC say there were 173,330 officially reported cases worldwide last year alone — though they believe that those numbers represent just a fraction of the actual number.

"In general, the number of reported cases reflects a small proportion of the true number of cases occurring in the community," the WHO has previously explained, saying the organization uses a statistical model to estimate the actual number. "Many cases do not seek health care or, if diagnosed, are not reported. In addition, there is a one to two month lag time in reporting."

In this case, the model estimates the actual number of cases last year to be 6.7 million.

Health officials believe they know the roots of the growth.

"Without urgent efforts to increase vaccination coverage and identify populations with unacceptable levels of under-, or unimmunized children, we risk losing decades of progress in protecting children and communities against this devastating, but entirely preventable disease," Soumya Swaminathan, the WHO's deputy director general for programs, said in a statement released Thursday.

Measles has menaced humans for more than a millennium. An infectious killer, capable of causing fatal complications such as pneumonia and encephalitis, particularly in children, measles has nevertheless faded from global fears in the five and a half decades since a vaccine was introduced in the U.S. In fact, the disease was declared eliminated in the country in 2000, and deaths from it have also fallen steeply worldwide since then.

But medical experts say those global successes have depended on the vaccine. Regions that do not have a high rate of vaccine coverage, whether due to a lack of access or conscious rejection by parents, are susceptible to a rise in measles — even relapses in areas where the disease had been nearly or entirely eliminated.

In the U.S., for instance, 220 measles cases have been reported so far this year as of Nov. 3, according to the CDC. There were 86 cases in all of 2016.

And the European Union has seen a measles outbreak that epidemiologists have tied to falling rates of vaccination, due to occasionally erratic vaccine supplies and anti-vaccine movements. In the first half of this year, the WHO says more than 41,000 children and adults in Europe were infected with measles — in other words, nearly double the cases recorded all of last year, which, in its own right, was the highest annual total in years.

The disease has also re-established a presence in other countries that had once been declared measles-free, such as Venezuela and Russia. "Since 2016, measles incidence has increased globally and in five of the six WHO regions," the organization notes, adding that only the Western Pacific region has avoided the larger trend.

"The increase in measles cases is deeply concerning, but not surprising," Seth Berkley, CEO of Gavi, the Vaccine Alliance, said in a statement released Thursday. "Complacency about the disease and the spread of falsehoods about the vaccine in Europe, a collapsing health system in Venezuela and pockets of fragility and low immunisation coverage in Africa are combining to bring about a global resurgence of measles after years of progress.

"Existing strategies need to change: more effort needs to go into increasing routine immunisation coverage and strengthening health systems," he added. "Otherwise we will continue chasing one outbreak after another."

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