Researchers looked at how the virus affected the blood of healthy men, pregnant women, and non-pregnant women aged 18 to 39. Much like HIV, researchers found that Zika targets the white blood cells that typically swallow up viruses, bacteria, and debris to keep the body healthy.

In expectant mothers, though, virus strains only had to infect 4% of white blood cells to increase immune suppression: An African Zika strain increased suppression by 10% and an Asian strain by 70%.

“Pregnancy naturally suppresses a woman’s immune system so her body doesn’t reject the fetus — essentially it’s a foreign object,” said senior study author Jae Jung, professor and chair of Keck’s department of molecular microbiology and immunology. “Zika exploits that weakness to infect and replicate.”

Prior research from the Centers for Disease Control and Prevention (CDC) found that one in ten pregnant women who contracted the virus gave birth to children with birth defects. And the risk for pregnancy complications is reportedly highest for women who contract the virus during their first or second trimester.

Interestingly, pregnant women are not involved in trials testing possible Zika vaccines, said Jung.

“The Zika virus vaccines in development seem to be highly effective, but they’re being tested among non-pregnant women with different body chemistry compared to pregnant women,” Jung said.

The mosquito-borne virus is still prevalent in Central and South America, the Caribbean, Central Africa, and Southeast Asia. The CDC maps countries with Zika and is still offering travel warnings and safety guidelines to consider for visits.

Precautions against the virus include preventing mosquito bites and practicing safe sex, but pregnant women and couples trying to conceive should not travel to areas with risk of Zika, according to the CDC.