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Chinese researchers published a report Thursday pinpointing when and how the Zika strain may have become dangerous for babies.

Researchers testing various samples from decades of strains, shows a single gene mutated three years ago to cause the devastating effects of the Zika virus we know today.

When virus genes mutate, some strains become innocuous, others become more dangerous to humans. 

The report says "ZIKV strains can be divided into the African and Asian lineages, and the contemporary strains circulating in Pacific islands and the Americas are likely to have evolved from a common ancestral strain in Southeast Asia. However, there is serological evidence that ZIKV had been circulating in Southeast Asia for many years.

"Why was microcephaly not recognized earlier? ...One plausible hypothesis is that ZIKV has acquired some adaptive mutations to become more virulent to the human fetal brain. .... Whether contemporary ZIKV strains are more likely to cause severe microcephaly in the fetus remains an open question."

According to the Center for Disease Control and Prevention website: Babies with microcephaly can have a range of problems, in addition to having an underdeveloped head and brain size, these babies suffer from seizures, vision problems, hearing loss and difficulty swallowing food.

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