THE BURNED OVER DISTRICT

After a great deal of forensic medical work, scientist think they have pinpointed the single mutation that changed Zika from an existing nuisance disease to a deadly virus responsible for thousands of children born with microcephaly.

Why did the Zika virus cause thousands of babies to be born with microcephaly, unusually small and damaged brains, when previous outbreaks in Africa and Asia seemed to cause much less harm?

An intriguing study in mice, which has prompted some skepticism among experts, suggests that a single genetic mutation helped transform the Zika virus into a devastating force in Latin America. The report was published on Thursday in the journal Science.

The mutation, called S139N, first arose in an Asian strain of the Zika virus in 2013, just before a small outbreak in French Polynesia — the first linked to an increase in babies born with microcephaly.

Zika is believed to have first appeared in Latin America later in 2013, possibly introduced by soccer players from French Polynesia competing in a tournament in northeastern Brazil. The mutation has appeared in every strain of the virus in the Latin American outbreak, the researchers said.

The study, by scientists in China, found that strains of Zika with the S139N mutation caused substantially more death and microcephaly in mice than other strains. And in a laboratory dish, the S139N strain killed many more human cells important to early brain development than an earlier strain without the mutation.

Some experts voiced doubts, saying the findings were too preliminary to establish that a single mutation was the critical factor. At least, they said (and the study authors agree), the results must be replicated in primates, because laboratory experiments with mice and even human brain cells cannot fully capture how the virus functions in nature.

Further research is needed to positively determine if this is the only cause, but it is frightening to realize that only one small bit of genetic material can have such a large effect.