Brazilian health authorities believe three adults died last year of suspected complications related to the Zika virus, the Health Ministry said on Thursday.

Researchers found the virus in the body of a 20-year-old woman who died in April from respiratory problems, a ministry spokesman said, confirming a report in O Estado de S. Paulo and Folha de S. Paulo newspapers.

The virus was previous detected in adults who died in June and October of suspected Zika complications, he said.

The little understood virus was previously believed to cause only minor symptoms, including a fever, rash and muscle aches, and often no symptoms at all. There is scant evidence in the limited studies on the disease of it being linked to fatalities.

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Brazil is also investigating a potential link between Zika infections and more than 4,000 suspected cases of microcephaly, a condition marked by abnormally small skulls in newborns and stunted brain development.

Researchers have identified evidence of Zika infection in 17 of these cases, either in the baby or in the mother, but have not confirmed that the virus can cause microcephaly.