The Zika virus has now spread to 20 countries in Latin America and the Caribbean, including some popular tourist destinations, and it's likely to spread farther, international health officials said.
And evidence is strengthening that the virus may cause a serious birth defect called microcephaly.
The virus can be expected to spread more, as the mosquitoes that carry it can be found across the region. It's now spreading locally in Barbados, Bolivia, Brazil, Colombia, Ecuador, El Salvador, French Guiana, Guadeloupe, Guatemala, Guyana, Haiti, Honduras, Martinique, Mexico, Panama, Paraguay, Puerto Rico, Saint Martin, Suriname, and Venezuela. Samoa, in the south Pacific, is also reporting Zika for the first time.
The U.S. Centers for Disease Control and Prevention on Friday repeated its recommendation that pregnant women avoid travel to affected countries until more is known about whether it can infect unborn babies.
But a team of Brazilian researchers published early results of a study of 35 babies affected in Brazil last year that strengthen the evidence. The babies were born in August through October and all had confirmed microcephaly, which causes underdeveloped heads and brains.
Microcephaly can be caused by a range of factors and two other babies were found to have other causes of their microcephaly — one had a virus called cytomegalovirus, and the other had a genetic disorder that can cause the condition.
The other 35 babies tested negative for syphilis, toxoplasmosis, rubella, cytomegalovirus and herpes simplex virus infections. Their spinal fluid is being analyzed for Zika and other infections, but the results are not back yet, the Brazilian team reported in a CDC bulletin. They said 26 of the mothers reported having had a rash during pregnancy. A rash is one symptom of Zika, which almost always causes a very mild infection.
"Further studies are needed to confirm the association of microcephaly with Zika virus infection during pregnancy and to understand any other adverse pregnancy outcomes associated with Zika virus infection," they wrote.
In a second report, a team of CDC experts said they're still not sure about the association.
They're also checking into reports of Guillain-Barré syndrome, a rare but serious neurological condition that can follow viral infections.
"Studies are under way to evaluate the risks for Zika virus transmission during pregnancy, the spectrum of outcomes associated with congenital infection and the possible association between Zika virus infection and Guillain-Barré syndrome," they wrote.
In the meantime, the only way for people to protect themselves is to avoid mosquito bites by using repellent, wearing protective clothing and staying indoors.
No one can yet say why Zika hasn't been associated with birth defects before, but the virus didn't start spreading widely until 2007. The European Centers for Disease Control says there are two strains of Zika — one from Africa and one from Asia. It's the Asian strain that's spreading in the Americas, the ECDC said in a review of what's known about Zika.
And last November, a team of Brazilian researchers found a genetic change in the virus spreading in Brazil. They're trying to discover if that change has made the virus more likely to spread and whether it's changed the way Zika affects people.