A child runs as a municipal pickup truck sprays a street with insecticide to kill the Aedes mosquito in Brasilia, Brazil, on Feb. 11. The mosquito can carry the Zika virus. (Adriano Machado/Reuters)

Brazil’s health ministry said Thursday that a 20-year-old woman infected with Zika has become the country’s third adult fatality linked to the virus, but scientists caution that they're only beginning to identify Zika’s potential risks to human health.

After falling ill last April, the woman began coughing up blood, and died after a 12-day hospitalization, according to Brazilian government researchers. The cause of death was registered as pneumonia, but her blood samples later tested positive for Zika.

Brazilian researchers said the patient’s respiratory problems were unusual for a case of Zika, so other factors could have contributed to her death. “She could have developed bronchial pneumonia and the association with the Zika virus made this worse,” said Pedro Vasconcelos, the Brazilian government scientist who led the tests.

The woman is among the first adult fatalities that health officials have attributed to Zika, which is transmitted primarily by mosquitoes and is projected to infect as many as 4 million people in the Americas this year, according to the World Health Organization.

Brazil’s government blames Zika for a sharp increase in babies born with abnormally small heads, a condition known as microcephaly.

There have been several deaths that are suspected to have a Zika connection. In Colombia, authorities last week said three Zika-infected patients had died of complications related to a temporary form of paralysis known as Guillain-Barré syndrome, which has been linked to the virus. Brazil has recorded two miscarriages by a Zika-infected mother and two cases of infants who had Zika and microcephaly, and died within 20 hours of birth, according to the U.S. Centers for Disease Control and Prevention. Researchers say they don’t yet understand whether the virus itself poses a mortal threat or how it might interact with other diseases or infections such as pneumonia.

“It is not possible to say that the Zika virus was the exclusive cause of death” in the most recently announced case, Claudio Maiorovitch, the Brazilian Ministry of Health’s director of transmissible diseases, told reporters Thursday. “We don’t know enough about the behavior of this virus in its interaction with the human body.”

One of the Zika patients who died in Brazil last year also had lupus. The other was a 16-year-old girl who had no serious health problems before being infected with Zika. Researchers don’t know yet to what extent Zika poses a threat to patients whose defenses are compromised by other diseases such as lupus, said Daniel Lucey, an infectious-disease specialist at Georgetown University Medical Center.

“If your immune system is weakened, a virus can get the upper hand and even cause death," said Lucey, who is in Brazil studying Zika. "That’s true for all infectious diseases.”

Brazil’s health officials acknowledged Thursday that they have been slow to develop a standardized, central record-keeping mechanism to track Zika’s spread, and said they have asked for additional international support. The government estimates that Brazil may have as many as 1.5 million Zika infections. But it does not know how many pregnant women have been exposed.

The virus has reached nearly three dozen countries, including the United States. On Thursday the CDC said two pregnant women had miscarried following their return to the United States after contracting Zika while traveling.

Although Brazilian authorities identified the Zika outbreak nearly nine months ago, the government has not required its hospitals and laboratories to report suspected or confirmed cases, saying they could not accurately test for the disease. Officials said they would begin distributing Zika test kits in the coming weeks and would start to require health workers to report infections.

“As the virus is new, we did not have the test to do the Zika diagnoses,” Brazilian Health Minister Marcelo Castro told reporters. “There was a little delay but it has been perfectly corrected.”

Other nations where Zika has spread more recently, including Colombia and El Salvador, have been publishing regularly updated statistics indicating how many people have been diagnosed with Zika, including the number of pregnant women.

Castro also announced Thursday a partnership with the University of Texas to develop a Zika vaccine. He said it could be ready for testing in a year and for mass production within two or three years.

The partnership developed after a telephone conversation between President Obama and Brazilian President Dilma Rousseff, Castro said.

He said 15 technicians from the CDC would arrive in Brazil this week, and that the government had also asked pharmaceutical giant GlaxoSmithKline for help developing a Zika vaccine. The company has previously worked with a Brazilian research institute on a vaccine for dengue, which also spreads primarily through infected mosquitoes.

The CDC technicians will study the relationship between Zika and microcephaly, officials say. Global health officials say they have a strong suspicion that the two are related.

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