Three people in New York City — including a pregnant woman — have been diagnosed with the mosquito-transmitted Zika virus after returning home from nations where the disease is flourishing, city officials said.

None are experiencing severe complications, and the cases are no cause for alarm, the officials said. But the city advised that women who are pregnant or might become pregnant should delay traveling to regions where there have been wider outbreaks, such as Central and South America.

Deputy Mayor for Health and Human Services Herminia Palacio and Health Commissioner Mary Bassett, both medical doctors, convened a meeting with City Hall reporters on the day the head of the World Health Organization warned that the virus is “spreading explosively” in the Americas. As many as 4 million people could be infected by year’s end.

Bassett declined to give details about the city cases, citing medical privacy rules.

“Travel to warm places in winter months is something that New Yorkers often look forward to doing,” Bassett said. While not saying people other than pregnant women should avoid tropical spots altogether, she said, “This might be a good winter to think about vacationing in the Catskills.”

Zika produces mild symptoms in most people, said Palacio.

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But the illness’ affect on pregnant women is particularly concerning because of the potential for microcephaly, a condition in which children are born with brain damage and abnormally small heads. As reported Zika cases rise in nations like Brazil, there have been an increase in reported microcephaly. Bassett urged pregnant women who have traveled to affected countries to consult their doctors.

Zika had not appeared extensively in the Western Hemisphere until the spring, and now millions in warmer climates have been diagnosed with the ailment. It usually goes away in several days to a week.

The kind of mosquito that transmits Zika in the Americas, Aedes aegypti, doesn’t live in the New York City area. But it’s theoretically possible for a person infected elsewhere to return to the city and — once the weather warms up — be bitten by a city-dwelling mosquito species, Aedes albopictus, who in turn could bite someone else and transmit Zika.

Officials say they’ll be keeping an eye on Aedes albopictus as the weather warms up, and adjust aerial spraying to suppress mosquito populations accordingly.