(CNN) In December, a lab at the Centers for Disease Control and Prevention received samples of brain tissue from two children who died within a day of birth and fetal tissue from two first-trimester miscarriages.
It's not uncommon for the infectious disease pathology lab at the CDC's National Center for Emerging and Zoonotic Infectious Diseases to be searching samples for something unknown. This time around, researchers there knew they were on a mission to find the Zika virus. The samples all came from women who had experienced suspected Zika symptoms while pregnant.
There were cases in Brazil of children born with small brains or microcephaly. Health officials there wanted to see if there was an association with the virus and the birth defect, said Dr. Sherif Zaki, chief of infectious disease pathology.
"(Brazilian Ministry of Health officials) were asking us very specific questions: Can you look at whether Zika virus is involved in terms of this microcephaly with small brains?" Zaki told CNN Chief Medical Correspondent Dr. Sanjay Gupta.
Zaki has been involved in almost every major infectious disease outbreak and discovery throughout his 27 years at the CDC: anthrax, hantavirus, nipah virus, SARS, MERS, Ebola, and now Zika virus.
He doesn't work alone. The effort requires pathologists, molecular biologists, epidemiologists and microbiologists. They consult with virologists, bacteriologists, epidemiologists and clinicians -- anyone who might have expertise. They look at slides and medical records from the patients, and talk through every last detail in hopes of solving whatever puzzle they are working on. As they rule anything in or out, they have to be certain.
In this case, before they could answer the questions from Brazil, they had to develop a test to identify the virus in the samples. They were able to do it within a month, according to Zaki. The test looks for proteins of the virus in the tissue.
When they ran the tests the first week of January, all four tissue samples from Brazil tested positive for the virus. "I couldn't believe them at first," Zaki said.
Previously, the virus had been found in amniotic fluid around the baby, but no one had found the virus in fetal tissue, he said. They did.
A moment like that is exciting and devastating, Zaki said. It's momentous to find something for the first time, but painful to know there are people grappling with the consequences of those findings, including people in Brazil whose babies will be born with microcephaly.
In this lab, the stakes are high and the pressure is immense. Zaki recalled the greatest pressure he'd felt in his career was in 2001, when he and his team identified the first case of anthrax. They were using a new test on skin biopsies that had tested negative in other labs previously. The CDC director at the time, Dr. Jeffrey Koplan, asked Zaki if he was 100% certain. Zaki remembers thinking he was only 90% sure.
He gave it more thought and came to his decision: It was, indeed, anthrax. It set off a chain reaction of media coverage and mobilized a response across New York and prophylactic prescriptions of antibiotics. For that reason, Zaki worries more about missing something or making the wrong call than about the devastating diseases he's seen under the microscope.
"You always have to keep in mind, 'could there be something else that could explain this' and explore those possibilities," he said.
For Roosecelis Brasil Martines, a pathologist in the lab, the research on Zika virus is personal. She is based in Atlanta, but she's from Brazil and was there for a conference last June, when the outbreak of the virus was starting to cause concern. Martines helped facilitate getting the tissue samples from Brazil to the CDC in December.
"I saw the situation in my own country," she said.
It's hard to be away from her home, she said, but she's glad to be helping from afar. She hopes the CDC can help, not only with research, but also with prevention and the development of a vaccine.
"I left my country to come here ... now I really can see results," she said, referring to the answers she and her colleagues have been able to find.
Evidence linking the mosquito-borne Zika virus to the birth defect microcephaly, other fetal malformations and possibly miscarriages is growing by the day. But the link is not yet conclusive. Experts continue collecting data to find out.
Since running those first samples from Brazil in January, Zaki and his team have run tests on tissue from 56 individuals with the virus. They are from Brazil, U.S. territories and across the nation.
Last month, Zaki showed CNN the virus from the placenta of a fetus that had been miscarried in the United States at eight weeks.
The mother had traveled to an area where the virus is circulating and returned home with symptoms, including a rash. This alone does not mean the virus caused the miscarriage, especially when one considers that 10% to 20% of pregnancies end in a spontaneous miscarriage, but the possibility is strong. The details of the case were familiar to Zaki, even though pathologists are usually disassociated from the clinical part of medicine, he said.
"I can sympathize with their concern," he said, referring to women who are worried about Zika virus. "If my daughter was pregnant [and had Zika] I would be worried."
With so many questions still unanswered, the work of this team is far from over and It's going to take time.
Until then, he said there needs to be a balance between the emotion and fear surrounding the virus, and the epidemiology and science.