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The explosion of cases of birth defects caused by Zika virus may be the "tip of the iceberg," experts said Sunday.

Many cases have probably been missed because babies looked normal when they were born. But hidden birth defects are almost certain to turn up as the babies grow, they told a meeting of pediatricians Sunday in Baltimore.

"The microcephaly and other birth defects we have been seeing could be the tip of the iceberg," Dr. Sonja Rasmussen of the U.S. Centers for Disease Control and Prevention said at the annual meeting of the Pediatric Academic Societies.

"The true burden of congenital disease with Zika virus is probably underestimated," said Dr. Marco Safadi of the Santa Casa Medical School in Sao Paulo, Brazil, who's been treating and studying cases.

Safadi described a case he just saw recently.

The baby looked OK when he was born in January, even though his mother had suffered some of the classic symptoms of Zika infection at the end of her second trimester of pregnancy: fever and a rash.

Related: WHO Warns Women on Travel to Zika Zones

But his head was just on the borderline of being normal, with a circumference of 12.7 inches. The cutoff for microcephaly, the small head that's the hallmark of Zika brain damage, is 12.9 inches.

A neurologist measures a child's head at Mestre Vitalino Hospital in Caruaru, Brazil, in December. Felipe Dana / AP

MRI and CT scans done when he was 2 months old showed that the baby had brain damage — areas of hardened brain tissue called calcifications, which happen when the cells die, and areas of the brain filled with fluid, a condition called ventriculomegaly.

"The microcephaly and other birth defects we have been seeing could be the tip of the iceberg."

The baby tested positive for Zika in his blood, urine and saliva. It was an unusual case, because the child was born in Sao Paulo, where Zika is not common. The father had been in northeastern Brazil a few weeks before the mother developed symptoms. "We think that probably a sexual transmission occurred," Safadi said.

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It's not yet clear what the consequences will be for the child. Newborns with microcephaly often act just like other newborns, perhaps a bit fussier. But the disabilities will appear as the growing children miss important milestones. They'll have learning deficiencies, vision problems and hearing problems, and many will also have physical disabilities.

There is no cure.

"We really don't know what will happen with these kids long-term," Rasmussen said.

Some of the cases are obvious and devastating, from very severe microcephaly, in which the head is abnormally small because of brain damage, to a horrific condition called fetal brain disruption sequence, when the overwhelming damage to the brain causes the baby's soft skull to collapse and take on a ridged appearance.

Related: How Can Zika Kill You?

Brazil has confirmed 2,844 cases of Zika in pregnant women.

Rasmussen said researchers have gone back to an outbreak in French Polynesia in 2013 and 2014 and estimated that 1 percent of pregnant women who became infected in the first trimester of pregnancy had children with microcephaly. Safadi said that would add up to many cases in Brazil, where 2.8 million children are born every year.

But there is some good news coming out of the early observations of the yearlong epidemic in Brazil. Children are rarely infected with Zika.

Safadi said clinics in Brazil are seeing just what French Polynesia saw in its 2013-14 outbreak: Patients have an average age of 42. Children and teens rarely get infected, and the researchers don't understand why not.

"We were screening for it and going out and even going out and looking for it, and we did not find any children with Zika infection," said Dr. Karin Nielsen, a professor of pediatrics at UCLA who is working with Brazilian colleagues at the Oswaldo Cruz Foundation in Rio de Janeiro.

But it does infect the fetus, and 54 miscarriages or newborn deaths have been caused by Zika, Safadi said.

The researchers have found that a paralyzing condition called Guillain-Barre syndrome is rare, hitting far fewer than 1 percent of patients: just 0.24 per 1,000. And Zika seems to cause a milder version of the condition, still putting patients into the hospital for 10 days or so, but not for as long as seen with other infections, Safadi said.

Zika rarely kills, but a man in Puerto Rico died last week from a hemorrhage caused by a rare immune reaction to the viral infection. It also causes a range of rare neurological disorders.

"We think that by August there will be little Zika circulating in Brazil."

And the researchers have another cheering prediction. The epidemic appears to be on the wane in Rio, at least for now, they said. By the time the Summer Olympics Games get under way in August, the peak of winter in Brazil, the risk of Zika and other mosquito-borne infections should be low, they said.

It supports what Brazilian Olympics officials have been saying. The Aedes aegypti mosquito that carries Zika is far more of a problem in the warmer, wetter summer months. August is usually sunny in Rio, with an average high temperature of 78 degrees.

Nielsen said her colleagues are already beginning to have trouble finding women infected with Zika to study.

"Zika is virtually disappearing from Rio," Nielsen said. "We think that by August there will be little Zika circulating in Brazil."

Safadi said, "That is exactly my perception."

But that won't mean it's gone.

Safadi thinks Zika may stay around for years, citing a study just published that showed monkeys have been infected with Zika in Brazil. That means they can act as a reservoir, as they do with yellow fever.

Even if a vaccine is developed and people get vaccinated, or if enough of the population gets infected to confer widespread immunity, the monkeys will be there for the mosquitoes to bite and carry the infection to people in the future. That will makes Zika a problem for years to come.

"We are trying to get rid of the Aedes, but I have to say we are failing in controlling this vector," Safadi said.