U.S. health officials issued a Zika virus travel warning on Thursday, recommending that pregnant women consider postponing nonessential travel to 11 counties in Southeast Asia.

The new travel warning was issued for Brunei, Myanmar (Burma), Cambodia, Indonesia, Laos, Malaysia, Maldives, Philippines, Thailand, Timor-Leste (East Timor) and Vietnam, according to the U.S. Centers for Disease Control and Prevention.

“Travelers have returned from certain areas of Southeast Asia with Zika virus infection,” the agency noted on its website.

On Friday, officials reported two cases of Zika-linked microcephaly in Thailand, the first confirmed cases of the birth defect in Southeast Asia. According to the Associated Press, Thailand has 349 confirmed cases of Zika virus, 33 of them in pregnant women.

The difference between Southeast Asia’s “endemic” and Latin America and the Caribbean’s “epidemic”:

It’s important to note like the map above that there’s a difference between the Zika “epidemic” sweeping South and Central America and the Caribbean ― for which officials issued a travel alert ― and the lower-level “endemic” in Southeast Asia, for which officials issued a travel consideration.

According to the CDC, Zika virus has been present in parts of Southeast Asia for years, and a large portion of the local population has likely developed immunity to the virus. Occasional Zika cases may occur, but it’s less likely that there will be a virus outbreak there than in virgin Zika territory. (Of course, travelers without such immunity may always be vulnerable.)

In comparison, Latin America and the Caribbean are experiencing a Zika epidemic. In those countries, the Aedes aegypti mosquito that primarily transmits the virus is present, and because there haven’t been reports of Zika virus in the area in the past, it’s unlikely that any local population has built up immunity. In Brazil, for example, where Zika virus spread rapidly this year, there have been 1,800 confirmed cases of Zika-related microcephaly.

Practically speaking, there’s not a big difference between the travel consideration and the travel warning. Both advise pregnant women or couples who are planning to become pregnant to consult a medical professional before traveling to an area with Zika virus, and to take steps to avoid mosquitos bites if they do decide to travel.

For babies exposed to Zika virus in-utero, the effects can be devastating. Infants born with microcephaly typically have abnormally small heads and can have development disabilities. And even those born with normal-sized heads can have damage from the infection, including brain abnormalities and joint malformation linked to the virus, according to a report published in August in the journal Radiology.