Three years ago, Brazil experienced a major sudden spread of the Zika virus.Doctors learned that the sickness can cause severe development problems for unborn babies.Here is a look at what scientists know todayabout Zika and its effect on fetuses.Zika began spreading in Brazil in April 2015.The country's Ministry of Health recorded more than 260,000 probable cases in 2016.It also recorded in the first two years more than 2,500 cases of babiesborn with very small heads, or microcephaly, and other problems linked to Zika.The numbers dropped sharply in 2017,with fewer than 18,000 Zika cases and 300 cases of microcephaly.So far this year, Brazil has seen about 2,200 cases of Zikaand 20 cases of microcephaly and other developmental abnormalities.Many viruses, like rubella and HIV, can cause microcephaly.Exposure to poisonous chemicals can also cause it.Sometimes microcephaly is the result of abnormal genetics.Since the 2015 outbreak, a group of experts organizedby the World Health Organization found that Zika is also a cause.But scientists noted that Zika appeared to lead to microcephalymore often in northeastern Brazil than it did in other places.They are still not sure why, said Ganeshwaran H. Mochida,a pediatric neurologist and researcher at Boston Children's Hospital.Scientists also wonder why an increase in microcephaly caseswas not reported during past Zika events.It could be that an increase was not noticed,or that the events were not large enough to produce many brain abnormalities.One study has suggested that a recent mutation to the virusmay have made it more likely to cause microcephaly.Mochida said, "This may not be the whole story...I don't think that this explains everything."Children with microcephaly can have substantial nervous system disabilitiesthat affect vision and hearing.Some will never learn to walk or speak.Scientists are still studying the effects of less severe cases.Microcephaly does not necessarily reduce a person's life expectancy,but it can lead to other problems that do.For example, many children have trouble swallowing,which makes them likely to breathe liquid or food into their lungs.This can lead to the lung disease pneumoniaZika has also been linked to other problems in babies, like seizures,restricted ability to move and poor balance.Scientists are still trying to learn if Zika may cause problems that appear later in a child's life,even for a baby who appeared healthy at birth.No vaccine is currently on the market,though several are being developed.Scientists at the U.S. National Institute of Allergy and Infectious Diseaseshave developed the best possible vaccine so far.It is being tested in 16 places in the United States, the Caribbean and Latin America.Dr. Anthony Fauci is director of the agency.He said approval for the vaccine could come in 18 monthsif scientists get data from a future outbreak.Barring that, approval could take three or more years.