The origins of the measles outbreak in the United States are not a mystery.
Persons infected with the virus brought it to the United States from Israel and Ukraine and passed it on to members of their communities, many of whom had not been vaccinated.
The challenge for US health authorities is stopping the outbreak from spreading further.
A total of 555 cases of measles have been recorded in the United States since January 1, most of them in New York and Clark County in Washington state.
In New York, an Orthodox Jewish community in Brooklyn has been hardest hit.
They were infected by visitors from Israel, where a measles outbreak began a year ago.
Last year, the disease spread through Brooklyn schools and synagogues among children who had never been vaccinated or had not had the recommended followup shot between the ages of four and six years old.
In addition to Brooklyn, another Orthodox Jewish community in upstate New York's Rockland County, has also been affected. About 200 cases have been reported there.
An outbreak in the state of Michigan – where 39 cases have been reported – has been traced to a single person, according to The Washington Post.
The Orthodox Jewish man arrived in the United States from Israel, spent time in Brooklyn and then drove to Detroit, where he spread the disease through visits to synagogues and markets.
Child from Ukraine
In Clark County, Washington, the measles outbreak is concentrated among a Russian-speaking community.
A child brought the virus back from Ukraine in December and it spread to 74 other people, mostly children, through schools, supermarkets and a bowling alley.
"This kid was in a pocket of kids that weren't immunized by choice," said Scott Lindquist, epidemiologist for the Washington State Department of Health.
"And that's how it started out.
"What we know about this cluster is they all match each other," Lindquist said. "We did DNA fingerprinting, or genetic sequencing, essentially.
"And they all match each other identically. And they all match the Ukraine strain," he said.
One theory links the outbreaks in Israel and Ukraine to an annual pilgrimage to the Ukrainian city of Uman by tens of thousands of Orthodox Jewish men to visit the grave of a revered rabbi.
Patrick O'Connor of the World Health Organization told The New York Times the major Ukraine outbreak may have supercharged a modest one in Israel.
Nancy Messonnier, acting director of the Center for Preparedness and Response at the Centers for Disease Control and Prevention, said all the US measles cases have come from other countries.
The number of cases in the United States are relatively few compared to other places but the second most since the turn of the century, when the disease was declared eliminated.
"We certainly are concerned that it will be the largest number of cases since elimination in 2000," Messonnier said
Tens of thousands of cases of measles have been reported in Africa and Europe. Ukraine alone has more than 30,000 cases and 11 deaths since January.
In the United States, the outbreaks have mostly been confined to tight-knit communities where vaccination rates are lower than the national average of more than 90 percent.
There have been outbreaks previously, for example, among the Amish community in Ohio in 2014 and Somali immigrants in Minnesota in 2017.
Health authorities have moved aggressively to prevent the disease from spreading.
In Clark County, 849 non-vaccinated students were barred from 15 schools where children had come down with measles.
"We were very aggressive about isolating any cases, quarantining anybody who was exposed to those cases, and excluding kids from school if they were unimmunised," said Lindquist, the Washington state health official.
"And that is what's slowed this outbreak down," he said. "Currently, we have not had a case in almost 42 days."
In New York, the mayor has ordered the vaccination of all residents of Williamsburg, a Brooklyn neighborhood, and threatened to impose fines of $1,000 for those who don't follow the order.
Rockland County, meanwhile, announced new measures to combat the disease, including fines of up to $2,000 a day.
"It is unacceptable to sit back and do nothing as more of our residents fall ill," county executive Ed Day said.
Anyone with measles or exposed to someone with measles was banned from public places.
Schools were ordered to keep out children who could not show proof of immunisation or a valid medical or religious exemption.