The Zika virus that created an international state of emergency just got more scarier: apparently, it can be sexually transmitted. Such a case was reported in the US.

Global Aedes aegypti predicted distribution. The map depicts the probability of occurrence (blue=none, red=highest occurrence).

The disease has been associated with thousands of babies being born with underdeveloped brains and has recently been spreading like wildfire through South America. Zika is generally transmitted through mosquito bites, like the related Dengue. However, a person contacted it without traveling to one of the infected areas, after their partner traveled to one of those areas.

“We certainly understand the concern. This needs to be further investigated to understand the conditions and how often or likely sexual transmission is, and whether or not other body fluids are implicated,” WHO spokesman Gregory Hartl told Reuters.

“This is the only the second mooted case of sexual transmission,” he said, referring to media reports about a case of an American man who returned from Senegal and is suspected of infecting his wife.

Anne Schuchat, principal deputy director for CDC, said this was the first case it had dealt with involving a “non-traveller”.

“We don’t believe this was spread through mosquito bites, but we do believe it was spread through a sexual contact.”

Fighting Zika is certainly not easy. The virus is helped by the same conditions as Ebola – warm temperatures, high humidity and poverty. The lack of hygiene is crucial for the spread of the disease, as well as for mosquitoes, which generally transmit it. For this reason, an outbreak in the US seems very unlikely, as mosquito control programs are already heavily in place. However, it’s not completely out of the question.

There are still many things we don’t know about the Zika – not even why the virus causes birth defects. Dr. Yoel Sadovsky, a Professor of Obstetrics, Gynecology and Reproductive Sciences, University of Pittsburgh said:

“We know very little about how Zika virus infection occurs during pregnancy and how it causes birth defects. However, what we do know from other viral infections during pregnancy is that there are several steps that are needed for viruses to affect the fetus. The first is to get into the mother’s body and then to infect or cross the placenta. At that point, the virus can enter a specific fetal compartment such as neurons which could potentially lead to a defect such as microcephaly. Alternatively the virus can remain in the placenta and may affect development of the fetus by disrupting placental function.”

It will take quite a while to figure these things out but in the meantime, the CDC advises pregnant women to avoid traveling to the infected countries (same for blood donors).