Up until now scientists have been focused on Zika and its effects on the brains of babies still developing in their mother's womb. In April, the Centers for Disease Control put up some definitive research saying the mosquito-borne Zika virus "is a cause of microcephaly and other severe fetal brain defects." But new research, published in today's Cell Stem Cell journal, says the virus may also damage some cells in the adult brain. Zika's broad impact on the human brain The new research was done by the Rockefeller University and La Jolla Institute for Allergy and Immunology and focused on special nerve cells in the adult brain called neural progenitor cells. These specialized cells are prevalent in the brains of developing fetuses and in adults. In a press release, the Rockefeller University calls progenitor cells "the stem cells of the brain," meaning they have "the capability to replenish the brain's neurons throughout its lifetime." It is thought that Zika attacks progenitor cells of babies in utero, and causes the reduced brain volume that is the hallmark of microcephaly. Scientists believe progenitor cells in adults have developed some resistance to Zika. That explains why the disease doesn't show up in adults, except – in most cases – as a brief, mild flu-like illness. Like baby, like parent The researchers had a hunch that if Zika affects the brains of developing infants, it likely will affect adults as well, so they decided to test their theory in mice whose brains had been engineered to mimic Zika infection in humans. "This is the first study looking at the effect of Zika infection on the adult brain," according to Joseph Gleeson, adjunct professor at Rockefeller. "Based on our findings," he said, "getting infected with Zika as an adult may not be as innocuous as people think." In mice, the progenitor cells live in two areas of the brain, and Gleeson found that in adults, that's where the Zika virus was also having an impact. "It was very clear that the virus wasn't affecting the whole brain evenly," Gleeson said. "In the adult, it's only these two populations that are very specific to the stem cells that are affected by virus." Those cells are thought to be critical to learning and memory. And what they also found was that when adults get Zika, more cells in the progenitor areas were dying and less cells were being regenerated. This means more research Sujan Shresta, a professor at the La Jolla Institute of Allergy and Immunology says their research suggests Zika is doing a lot more to the brains of babies and adults than previously thought. "… It's a complex disease. It's catastrophic for early brain development, yet the majority of adults who are infected with Zika rarely show detectable symptoms. Its effect on the adult brain may be more subtle, and now we know what to look for." Gleeson says that most adults seem to be able to overcome any effects of Zika, but that people with compromised immune systems could be more vulnerable to some of its negative effects. "In more subtle cases, the virus could theoretically impact long-term memory or risk of depression," he says. And a lot more research is necessary in order to prove that Zika is having the same effect in adult human as it does in mice. At this point there are more questions than answers, but the possibility that Zika could have an effect on adults increases dramatically its potential health impacts as the virus continues to spread.