Scientists have made a discovery regarding the Zika outbreak that suggests the virus may be tough to stop.

Scientists at Emory University have found a strong link between sexually active women and infection rates of the Zika virus that indicate that sexual transmission may be the most likely way this disease is spreading, according to a Cell Press statement.

It’s an alarming finding that both provides hope of finding a better way to treat it and also shows it may be quite difficult to stop.

Researchers examined Zika infections based on the age of those who get the virus, and found that women who were at sexually active ages between 15 and 65 were 90 percent more likely than men of the same age.

The Zika virus is spread by mosquitoes and can cause miscarriages or stillbirths, even though symptoms in people who actually have it are usually quite mild. The outbreak began in South American but has quickly spread northward and has made its way into the United States.

“One group has recently discovered viral antigen in Hofbauer cells collected from placental tissue of a fetus that unfortunately died as a result of Zika virus infection,” says senior author and Assistant Professor in Pediatrics at the Emory University School of Medicine, Mehul Suthar, in the statement. “Our study indicates that this cell type may be a target for Zika virus in the placenta and replication in these cells may allow the virus to cross the placental barrier and enter the fetal circulation,” adds co-author Rana Chakraborty, a pediatric infectious disease specialist, also at Emory.