Concern over the Zika virus - by now, the U.S. has had more than 50 cases, all associated with travel - has brought new focus to a birth defect that has been linked to the illness.

It is microcephaly, which already affects about five in 1,000 children.

David Bearden, a clinical instructor at Children's Hospital of Philadelphia, has focused much of his research on congenital infections that can cause microcephaly. Bearden, also medical director of the International Program for Neurology at CHOP, spoke to us recently about the condition.

What is microcephaly?

Microcephaly literally just means "small head," and usually that's defined according to growth curves. If a child's head is below the third percentile - smaller than 97 percent of the population - that's how we define microcephaly. Usually, it's something that would be picked up either when the child is first born, or later on by the pediatrician.

It can be caused by essentially anything that damages the brain. Some of the most common causes are congenital infections, especially ones that infect a fetus in utero. It can also be caused by the fetus' not getting enough oxygen. The third thing that often causes microcephaly is genetic disorders.

The health community has started to accept that there is now strong evidence that the Zika virus can cause microcephaly. There's not a whole lot of direct evidence, but there's a lot of indirect evidence. So people have come to think that the Zika virus is at least a potential cause of microcephaly, or is causing some cases of microcephaly. What we don't know is how often that happens and to what extent the microcephaly increase rates we're seeing in places like Brazil is due to the Zika virus.

How many children does it affect?

The rate of microcephaly in the U.S. and Western Europe is about five out of every 1,000 children. The rate is higher among premature babies. The rate also is probably higher in low-resource settings, such as Africa and South America, but we don't know for sure. This is actually one of the problems in Brazil right now. They weren't reporting microcephaly very regularly in Brazil up until the Zika virus hit. Then, all of a sudden, they started measuring everyone's heads closely. And finding lots of microcephaly. One of the challenges is we don't know how much of the microcephaly is caused by the Zika virus and how much is simply because they're paying closer attention.

What is the likely outcome for a child with microcephaly?

The main reason microcephaly is a problem is because it's a reflection of brain injury. In and of itself, it's not a problem. But it's a sign that something has gone wrong with the brain.

The outcome really depends on the underlying cause, how much of the brain was injured, and what parts of the brain were injured. Some children with microcephaly, especially in genetic cases of microcephaly, will have normal intelligence and really won't have any problems. But in other cases, where the microcephaly is caused by congenital infections, the outcomes can be significant and include things like mental retardation and a high risk of epilepsy.

The more important thing, rather than the size of the head, is the underlying cause. If someone has a child with microcephaly, it's important to talk to the pediatrician about what's causing that, and whether any further evaluation is necessary.

Most of the time, we can figure out the cause. Not always. But usually, with a combination of brain imaging and, in the right circumstances, genetic testing, we can determine the underlying cause.

How is microcephaly managed?

We just try to support the patient for the other conditions, such as intellectual difficulty or cerebral palsy. There's not anything we can do specifically for the microcephaly.

What's next? What are the big unanswered questions about microcephaly?

Some of the questions about microcephaly have to do with the underlying causes, especially viral infections. So here, the relationship between Zika virus and microcephaly is something that has to be studied further. One thing we don't understand is how often Zika virus results in microcephaly. Right now, we know that most people who have Zika virus don't have any significant problems.

The second thing we don't understand is how the Zika virus causes microcephaly. We think it probably gets into the fetus and injures the brain in some way, but we really don't know how that works right now.

A lot of research is just getting started on Zika. This is a virus that has been around a long time; it was discovered in the 1940s. But it's only recently that it has come to our attention as an important virus for humans. Over the next year, we'll get a lot more information.

Things are changing rapidly, even in just a matter of weeks. More and more information is coming out. What we think is true today might be different tomorrow. This is something that pregnant women considering travel to countries that might involve exposure to Zika virus should take into account. The recommendations have been changing even just week to week in terms of what precautions people should take and what places pregnant women should avoid going to.