The CDC just issued a special Zika travel advisory for 11 Southeast Asian countries

On Thursday, the Centers for Disease Control and Prevention issued a special Zika travel advisory for 11 Southeast Asian countries.

"CDC now recommends that pregnant women should consider postponing nonessential travel to these countries because of the uncertain risk of Zika virus infection," the agency said in a statement.

The countries on this list are: Brunei, Myanmar, Cambodia, Indonesia, Laos, Malaysia, Maldives, Philippines, Thailand, East Timor, and Vietnam.

The Zika virus, which is mainly transmitted by mosquitoes, has been spreading around the Americas and parts of Asia over the past year. It's been identified in the mainland US in Miami, Florida. There have been outbreaks in Southeast Asia for a number of years, and, the CDC noted, many residents are likely immune. It's travellers coming from the US that the CDC is concerned about, especially since "recent variations have been observed in the number of cases reported" in the region. (That could be due to multiple factors, including higher awareness of the virus or a more infectious strain.)

The CDC emphasized that "the level of this risk is unknown and likely lower than in areas where Zika virus is newly introduced and spreading widely."

Once infected with Zika, only about 20% of people ever show symptoms, which most commonly include fever, rash, joint pain, and red eyes.

One reason Zika is troubling is that it is a cause of birth defects, including microcephaly - a condition where the baby's head is abnormally small - in babies whose mothers have had Zika. The virus has also been linked to a neurological condition called Guillain-Barré syndrome.