CMV should be every bit as urgent a priority as Zika, he argues. Health officials called for a vaccine decades ago, and there still isn’t one, partly because of a lack of public awareness about CMV, Dr. Schleiss said.

CMV is a hardy member of the herpes family, and it is transmitted by contact with saliva and urine — often from diaper-wearing children to adults. Pregnant women often get it from toddlers, especially those in day care who share drool-drenched toys.

“Toddlers are hot zones for CMV,” said Dr. Gail Demmler-Harrison, a pediatric infectious disease specialist at Baylor College of Medicine in Houston. It is difficult for mothers to protect themselves from a virus carried by the children they care for.

Nearly one in three children is infected by age 5, and more than half of adults by 40. CMV takes up permanent residence in the body and can cause illness again after being dormant. Like the Zika virus, it causes mild flulike symptoms, or none — but can be devastating to a fetus.

Had she known any of that while pregnant, Ms. Sweet might have reduced her chances of contracting CMV with diligent hand-washing, especially after diaper changes, and not sharing utensils or food with her son, Henry, then 2 and in day care.

“If there was awareness about CMV, at least women working in day care and women with toddlers could potentially modify some behavior,” Ms. Sweet said.