SALT LAKE CITY — Epidemiologists in Utah have long-awaited the first frost — usually in mid-October — as a turning point for when the risk of acquiring a mosquito-borne disease drops significantly.

But with the Zika virus still potent in many warmer countries, health officials say the cold weather doesn't diminish the threat of acquiring the virus abroad.

"We're still testing people every day," said Amy Nance, program manager of the Utah Birth Defect Network at the Utah Department of Health, cautioning Utahns to continue to heed public health warnings about travel to Zika-affected areas.

"People are still traveling, especially to areas where Zika is still very much a threat," Nance said. "So the concern is we don't know how many people are traveling to these areas."

Eighteen people in Utah have tested positive for the mosquito-borne virus, according to the health department, including one who tested positive last week.

Of those 18, eight were or are pregnant women.

Four of the women have given birth to healthy babies, according to Nance, and two have been lost to miscarriage.

Nance declined to say whether those pregnancies showed signs of birth defects for privacy reasons, but she noted that researchers are investigating an increased risk of pregnancy losses associated with Zika virus.

She said health officials will closely track the health of all babies born to Zika-infected mothers for up to a year.

The most common birth defect linked to the Zika virus is microcephaly, a condition in which babies are born with unusually small heads and underdeveloped brains. Research shows that some babies born to Zika-infected mothers may appear healthy at birth but later show developmental problems.

In an essay for the Journal of the American Medical Association published Tuesday, Centers for Disease Control and Prevention Director Thomas Frieden worried that "concern among the general public has been muted even in some affected locales.”

Frieden stressed that Zika continues to represent an "unprecedented emergency" for the public health community.

"Never before, to our knowledge, has a mosquito-borne virus been associated with human birth defects or been capable of sexual transmission," Frieden said.

Experts are continuing to urge pregnant women not to travel to Zika-affected areas even as many Americans plan winter getaways to tropical locales.

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Couples who are trying to get pregnant should continue to avoid nonessential travel to Zika-affected areas, according to the CDC. That includes most places in the Caribbean, South America, Central America and parts of south Florida.

The public health organization also started advising pregnant women to avoid parts of Southeast Asia last month.

Because the disease can be sexually transmitted, the CDC also urges men who have recently traveled to a Zika-affected area to abstain from sex with pregnant partners for the duration of the pregnancy.

People who are infected with Zika should abstain from sex for at least six months after the onset of the illness, the CDC said.

No vaccine exists to prevent Zika, which is primarily spread by a species of mosquito that is prevalent in many warmer parts of the world but not in Utah.

The scale of the problem continues to grow in the U.S. and abroad, with more than 3,800 cases in the U.S. as of last Wednesday — including 837 pregnant women.

Utah health officials also warned that West Nile virus remains a concern until the first frost, although mosquito numbers are starting to decline.

The Utah Department of Health recorded 13 human cases of West Nile in Utah this summer, including two deaths.

Both of those individuals had underlying health issues, according to Utah Department of Health epidemiologist Dallin Peterson.

Flu will also become a greater concern as the winter months approach. Health officials continue to encourage people to get flu shots.