CHICAGO (Reuters) - U.S. disease detectives have moved into full outbreak mode over the Zika virus, assembling a team of hundreds of experts to try to better understand its impact as it spreads in the Americas.

A sterile female Aedes aegypti mosquito is seen on the forearm of a health technician in a research area to prevent the spread of Zika virus and other mosquito-borne diseases, at the entomology department of the Ministry of Public Health, in Guatemala City, January 28, 2016. REUTERS/Josue Decavele

On Sunday, the U.S. Centers for Disease Control and Prevention dedicated an emergency operations center staffed around the clock to address Zika, a mosquito-borne virus linked to severe birth defects in thousands of babies in Brazil, agency officials told Reuters.

The CDC has set up such command centers to handle the Ebola outbreak in 2014 and the Haiti cholera epidemic that began in 2010. This time, the team includes many more experts on pregnancy given the unusual impact of Zika.

“We always involve OB-GYNS and pediatricians. In this response, this team is actually much larger and is in many ways the focal point of the activities,” Tracee Treadwell, who is helping to lead the CDC’s Zika response, said in an interview.

The World Health Organization on Thursday said it would consider next week whether to declare Zika an international health emergency, and estimated that as many as 4 million people could be affected by the virus as it spreads in Latin America and the Caribbean to North America in the coming months.

The U.S. public health agency has been working with the Brazilian Ministry of Health and the WHO since early November to understand the sudden increase in cases of microcephaly, a birth defect marked by small head size that appears linked to Zika. Brazil said this week about 3,700 cases of microcephaly are being studied for signs of Zika.

“It’s an extraordinarily complicated situation. There appears to be an association, but we just don’t have enough data yet,” Treadwell said.

The agency grew more confident of a link between microcephaly and Zika in the middle of January, after it conducted its own tests on tissue samples from two stillborn babies and two babies who died shortly after birth.

All four of the microcephaly cases from Brazil were positive for Zika. A genetic analysis showed the virus from the tissue samples matched the strain circulating in Brazil.

The test results prompted the CDC’s travel warning on Jan. 15, which advised pregnant women to avoid affected countries.

The agency is working with Brazil’s government to set up a rigorous study of Zika’s association with microcephaly and is already collaborating on research into a link with Guillain-Barre, a rare disorder in which a person’s immune system attacks nerve cells.

All Zika communications go through CDC Director Dr. Thomas Frieden and Dr. Lyle Peterson, who is heading the U.S. response.

The CDC has received “a lot of requests for assistance in many other countries” beyond Brazil, said Treadwell, one of two deputies reporting to Peterson. “We’re in the throes of working on what sorts of assistance can we provide.”