U.S. health officials are advising pregnant women to postpone travel to 11 countries in Southeast Asia because of Zika outbreaks in the region.

The advisory issued Thursday targets travel to Brunei, Cambodia, East Timor, Indonesia, Laos, Malaysia, the Maldives, Myanmar, the Philippines, Thailand and Vietnam.

The Centers for Disease Control and Prevention said Zika has been in some areas of Southeast Asia for years, and some residents may be immune. But a number of U.S. travelers have become infected there in the last year, so there is a danger to visitors.

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Most infected people suffer a mild and temporary illness, at worst. But infection during pregnancy can causes severe brain-related birth defects. The virus is spread primarily by bites from infected mosquitoes.