Scientists say their study provides a potential explanation on how the Zika virus might cause serious brain defects in unborn children.

Researchers may have uncovered an important clue to explain why the Zika virus could be responsible for a serious brain defect in newborn babies.

In a study published today in Cell Stem Cell, researchers from Johns Hopkins University School of Medicine, Florida State University, and Emory University say the Zika virus infects a type of neuronal stem cell associated with the brain’s cerebral cortex.

In their experiments, the researchers said these stem cells became “havens for viral reproduction.” This process led to cell death and disruption of cell growth.

The researchers said the Zika virus spread from a single infection through a dish of stem cells in just three days. There was no evidence the cells employed any antiviral responses.

The scientists said their discovery doesn’t yet prove a direct link between Zika and microcephaly, but it does pinpoint where the virus may be doing its most damage.

“This is a first step, and there’s a lot more that needs to be done,” Hongjun Song, a neuroscientist and stem cell biologist at the Johns Hopkins facility said in a statement. “What we show is that the Zika virus infects neuronal cells in [a lab] dish that are counterparts to those that form the cortex during human brain development.”

Read More: Researchers Say Mosquito Gut Bacteria Can Be Used in Fight Against Zika Virus »

Dr. Lee Norman, chief medical officer for the University of Kansas Hospital, said the study seems to confirm earlier research that the Zika virus is a culprit in microcephaly.

“It appears to take it out of the realm of conjecture,” Norman told Healthline.

Norman added it also appears the virus infects the neuronal cells only in fetuses. For whatever reason, it doesn’t do that when it invades an adult or even a young child’s body.

Most adults and children who are infected by Zika don’t even exhibit symptoms. Those that do usually only have mild flu-like symptoms.

Norman said viruses have different mechanisms and sometimes thrive only in some parts of the body or on certain types of systems. The characteristics are known as tropism.

The West Nile virus, for example, has been known on rare occasions to cross the blood-brain barrier and cause serious neurological illnesses.

In Zika’s case, it appears to do serious damage only in developing fetuses.

Read More: Are Mosquitoes the Most Dangerous Animal on Earth? »

In the near future, the Johns Hopkins researchers hope to grow mini-brains from the stem cells to observe the long-term effects of the Zika infection in developing cells.

“We hope our results will help educate the public and government decision makers because they need to have more information on this virus, and we have to take it seriously,” Song said.

In another study, researchers in Brazil said a mosquito species more common that one known to carry Zika may be capable of transmitting the virus.

The researchers said they were able to infect the Culex quinquefasciatus mosquito with the virus in a laboratory. Up until now, only the Aedes aegypti species was known to be a carrier.

The Culex species is 20 times more common than the Aedes mosquito.

Read More: United States Warned to Brace for Zika Virus »

On Thursday, a fifth case of Zika virus infection was confirmed in North Carolina. An initial case of Zika virus was also confirmed in Oklahoma.

Also this week, a pregnant woman in California’s Napa Valley was diagnosed with the Zika virus. It was the sixth confirmed Zika infection in California.

The spread of the virus is serious enough that the World Health Organization (WHO) on Thursday issued a new set of guidelines for pregnant women living in or traveling to warm climates, poor communities, or other areas where mosquitoes thrive.

Among the information was a warning from WHO officials that pregnant women are just as likely as the general population to be infected with Zika. In addition, those carrying the virus may not develop symptoms, so they might be unaware they have been infected.

The U.S. Centers for Disease Control and Prevention (CDC) also has a section on their website with recommendations and information on the Zika virus and pregnancy.

Chloe Demrovsky, the executive director at the Disaster Recovery Institute, said there are number of other ways people can lower their risk of contracting Zika.

The most obvious is to reduce their exposure to mosquitoes.

In an email to Healthline, Demrovsky said people can use insect repellant and protective clothing to reduce the chances of being bitten.

Buildings with screens and air conditioners can also help keep mosquitoes out.

The removal of standing water is also an effective preventative measure. Demrovsky noted that the Aedes mosquito can lay eggs in as little as a tablespoon of water.