It was just last month that the first case of a woman spreading Zika sexually to her male partner was documented, and around the same time that a study found the virus in a woman’s cervical mucus 11 days after infection. Before that, most of the talk about sexual transmission of the normally mosquito-borne virus focused on semen, where Zika can survive for a long time. (A recent report puts it at up to six months.)

But it’s starting to look like the vagina is very hospitable to Zika as well. Which is alarming, given the severity of the birth defects Zika can cause. So far there are only case studies in humans, but a new study in mice found that the female reproductive tract is “a highly susceptible site of ZIKV replication.”

The thing about mice is, they’re not very susceptible to Zika generally. If you take what researchers call “wild-type mice,” meaning they haven’t been genetically altered, and inject them with the virus like a mosquito would, “[it] doesn’t replicate very robustly,” says Laura Yockey, the lead author on the new study and a graduate student at Yale University.

Normally, a mouse’s natural immune response will take care of Zika. Not the adaptive immune response, which is when a body develops antibodies to a specific pathogen, but the innate immune response, which is a more generalized response triggered by interferons—proteins that signal cells to get into fighting mode. In humans, Zika appears to mess with the interferon response, but not in mice. So most research on Zika in mice has had to be done with mice that are genetically altered to have diminished interferon responses.