The Zika virus is traveling across the U.S. and around the globe, spreading fears about microcephaly and other birth defects in developing fetuses whose mothers are infected with it. But how does it get inside us in the first place?

What We Know

The Zika virus, like malaria and dengue fever, is primarily spread by the bite of a female mosquito, particularly those belonging to the Aedes family.

Since its arrival on the world stage, however, scientists have become aware that it can occasionally spread through sexual contact. At first, it was assumed that only male carriers could give to their partners, but in recent weeks, there’s evidence that women can also spread Zika to their male counterparts.

In light of this, health organizations like the Centers for Disease Control and Prevention have issued warnings against travel to Zika-infested areas for pregnant women (including Florida now), and recommended that anyone who’s traveled to these places abstain from sex or use protection for at least eight weeks after returning, even if they don’t show symptoms (though dangerous to pregnant women and their children, the vast majority of sufferers experience little sickness).

What We Don’t Know

Earlier this July, the son of an elderly man in Utah who died from Zika contracted the disease as well, despite not having traveled recently. And unlike the local cases of Zika in Florida, Utah does not have the type of mosquitoes known to spread it, ruling that out as well. CDC researchers are still at work trying to figure out the mystery.

In Brazil, where the outbreak first gained widespread attention, Zika has also recently been detected in Culex mosquitoes, a much more common type of mosquito found throughout the U.S. Scientists don't yet know whether these mosquitoes are even capable of spreading the infection. These mosquitoes are found in Utah, but since there’s been no other cases in the vicinity of the original two infections, they’re unlikely to be the culprit.

Lastly, while researchers are not sure that Zika could be transmitted by donated blood, it’s more than likely that it can. The Food and Drug Administration has advised that all blood centers in Florida’s Miami-Dade County cease collection until they’re able to effectively screen for the virus.