ATLANTA -- Ear infections, a scourge that has left countless tots screaming through the night, have fallen dramatically, and some researchers suggest a decline in smoking by parents might be part of the reason.

Health officials report nearly a 30 percent drop over 15 years in young children's doctor visits for ear infections. That's half a million fewer trips to the doctor on average.

Why the numbers are declining is a bit of a mystery, but Harvard researchers think it's partly because fewer people smoke, meaning less irritation of children's airways. Many doctors credit growing use of a vaccine against bacteria that cause ear infections. And some think increased breast-feeding is protecting more children.

"We're sort of guessing here," said Dr. Richard Rosenfeld, a New York-based ear, nose and throat specialist who speaks about the issue for the American Academy of Pediatrics.

To be sure, middle ear infections still plague many U.S. children.

For decades, they were the most common reason parents brought young children to a doctor, according to health officials. The Centers for Disease Control and Prevention hadn't bothered to issue a report on them in nearly 20 years.

Cases skyrocketed from 1975-1990. The visit rate for children 5 and under more than doubled in that time.

A big reason, Rosenfeld said, was a steady rise in dual-career families. More families put their kids in day care, and day care is a breeding ground for the germs that lead to ear infections.

But the study by Harvard University suggests another contributor: cigarette smoke.

Most ear infections occur after a cold. In children, the ear is more directly connected to the back of the nose, so infections in a child's nose and throat can easily trigger ear inflammation. Such swelling is a fertile setting for the bacteria that cause ear infections.

Cigarette smoke, inhaled through a child's nose, can trigger the same kind of irritation and swelling, said Dr. Gordon Hughes of the National Institute on Deafness and Other Communication Disorders.

CDC figures show that 88 percent of U.S. nonsmokers were exposed to secondhand smoke around 1990, but that fell to about 40 percent in 2007 and 2008.