Back in 2015, Brazil reported a horrific a surge in birth defects. Thousands of babies were born with ... a condition called microcephaly. Scientists quickly concluded the Zika virus was the culprit. So when Zika returned last year during Brazil's summer months ... health officials expected another surge in microcephaly cases. But that never happened. We apparently saw a lot of cases Zika virus in 2016. But there was no microcephaly," says Christopher Dye of the World Health Organization. The difference between 2015 and 2016 "is spectacular," he says. Health officials were predicting more than 1,000 cases of microcephaly in the northeast of Brazil last year. But there were fewer than 100, Dye and his colleagues report Wednesday in the New England Journal of Medicine. "This is a huge, huge discrepancy," Dye says. "So what could possibly be the explanation for that?" Scientists aren't sure, Dye says. But he and his colleagues suggest a few possibilities in their study. First off, Dye says, health officials could have vastly overestimated the number of Zika cases in Brazil. Another possible explanation: Zika might not be working alone. Maybe another infection combines with Zika to make the disease worse and increase the risk of birth defects.