The virus spreads through the air and can linger for up to two hours in an isolated space.
That helps explain why public health officials get so concerned when outbreaks occur, which they have been with increasing frequency in recent years.
Clark County also listed 11 suspected cases of measles.
Meanwhile, Ukraine had the highest rate of measles cases in Europe, at 1,209 per one million population - 10 times the country's rate in 2017.
Measles is one of the most highly contagious viruses in existence, which makes the prospect of an outbreak frightening.
Complications can include otitis media, pneumonia, encephalitis, seizures and death.
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Across the US, 10 states - California, Colorado, Connecticut, Georgia, Illinois, New Jersey, New York, Oregon, Texas and Washington - reported 79 confirmed measles cases in January, according to the Centers for Disease Control and Prevention.
A total of 72 children and adults died in 2018 following infection, the World Health Organization said.
A person can carry measles for up to two weeks before symptoms show.
Washington is one of many states with non-medical exemptions.
Measles, a highly contagious viral disease common among children, causes a rash and high fever.
While families in Madagascar have difficulty accessing the measles vaccine, outbreaks in Europe and the USA are often linked to vaccine hesitancy - the reluctance of parents to vaccinate their children because of unfounded fears about the treatment.
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Clark County Public Health Department Public Information Officer Marissa Armstrong said that the first case in the Clark County outbreak was a child who had traveled to Clark County from outside the country - information that was announced in the initial outbreak news release January 4. Orders of measles vaccines in the county reached 3,150 in January.
Children can be vaccinated once they are 12 months old.
"This outbreak has put people at real risk", said Ann Thomas, public health physician at the Oregon Health Authority.
On Wednesday, officials sent letters to families of 5,000 children in Multnomah County, telling them they'll be excluded from school if they don't have up-to-date immunizations or valid exemptions by February 20.
In 34 of the countries, estimated coverage with a second dose of measles vaccine was below 95 per cent.
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