The potential link between the Zika virus and brain malformations in babies is terrifying pregnant women around the world. But even in places not hit by the virus, kids are at risk of being born with microcephaly, or a smaller-than-average brain.

Many of those cases are caused by a virus you’ve probably never heard of: Cytomegalovirus or CMV. And unlike Zika, which is currently only being actively transmitted in Latin America and a few other dots on the world map, CMV exists virtually everywhere (pg. 3.)

Yet as Zika grabs headlines, CMV remains virtually unknown among the general public, says Gail Demmler-Harrison, an infectious-disease expert who researches the virus at Texas Children’s Hospital.

“The psychology of humans is to be reactive rather than proactive,” she tells Quartz. “If there’s an outbreak such as with Zika or Ebola, people react and there’s a lot of interest.”

In the US, about 1% of the 4 million babies born every year are infected with CMV, per the Congenital CMV Disease Research Clinic and Registry at Baylor College of Medicine in Houston. While most of them—about 90%—won’t show any symptoms, the rest may have at least one of a variety of abnormalities, including hearing loss and microcephaly. That’s around 4,000 affected babies.

For perspective, only 17 of the roughly 400 microcephaly cases confirmed by Brazilian health officials so far have conclusively tested positive for Zika infection, according to the latest data (pdf, pg. 3) from the World Health Organization. Whether the virus was responsible for the condition or just coincidentally there is a question scientists around the world are still struggling to answer. (To be sure, officials in Brazil still have more than 3,500 suspected cases to investigate, though until now more than 60% of the 1,100 or so they’ve already checked turned out not be microcephaly.)

CMV, which has been studied for decades, is transmitted through bodily fluids, such as saliva and breast milk. In the US, 50% to 85% of adults will be infected with it by the time they reach 40, according to data from Baylor’s CMV clinic. In other countries, that happens even earlier, during childhood or the teenage years. Because it generally doesn’t cause any symptoms, many people never realize they’ve been infected.

But the virus can cause serious damage when transmitted from mother to unborn child. More awareness about CMV could help avoid that. Due to their less-than-stellar hygiene habits, small children can easily get infected with the virus after birth. Although they personally don’t suffer any ill effects, they become “a virtual hot zone for CMV” for pregnant women who interact with them, according to Demmler-Harrison. A few simple practices, such as not sharing food with toddlers and handwashing after changing a diaper, can help mothers-to-be avoid catching the disease from little ones.

Detecting CMV infections early also provides more options to treat the disease. For example, certain antiviral treatments might help minimize hearing loss, she adds.

Advocates for the prevention of the disease are vying to take advantage of the global attention on Zika to shine the spotlight on CMV. The National CMV Foundation, for example, is sharing an infographic on social media that compares the two viruses.