The Centers for Disease Control (CDC) announced that doctors had confirmed the presence of Zika virus in the tissue of infants who died from microcephaly, a rare birth defect.

“This is the strongest evidence to date that Zika is the cause of microcephaly,” explained CDC Director Tom Frieden. “Zika is new, and new diseases can be scary, particularly when they can affect the most vulnerable among us.”

Frieden stopped short of claiming Zika caused the microcephaly, a disorder in which the brain does not form properly during pregnancy or after birth and results in a small head. He said scientists need to perform more tests to confirm an actual link.

The CDC stated the disorder is typically uncommon. Following the Zika outbreak, however, Brazil has documented over 4,000 cases and is diagnosing an average of 200 cases a week. In 2015, the country tracked over 2,400 cases, compared to the 147 in 2014.

Doctors reported 404 cases of microcephaly in newborns since November. They tied seventeen “to the Zika virus.” Fifteen of the 404 passed away, “with five linked to Zika.” Authorities are investigating 56 other deaths and 3,670 cases.

Aedes aegypti mosquitoes carry the disease. While primarily found in Africa, all but two nations in the Western Hemisphere have recorded populations of the mosquito species living there (Chile and Canada are the exceptions). They also carry dengue, yellow fever, and chikungunya. Doctors found Zika in one infant during an autopsy, and numerous mothers reported symptoms.

Brazil, El Salvador, and Colombia have advised women against getting pregnant.

“It’s a very personal decision, but at this moment of uncertainty, if families can put off their pregnancy plans, that’s what we’re recommending,” stated Angela Rocha, the pediatric infectologist at Oswaldo Cruz Hospital in Brazil.

Yale University School of Public Health Professor Albert Ko claims the cases of microcephaly in Brazil are just the start of concerns about birth defects linked to the Zika virus.

“It seems like microcephaly may just be the tip of the iceberg,” he stressed, adding:

The preliminary evidence is that [some] babies who don’t have microcephaly may also have neurological lesions or birth defects that are not as obvious as microcephaly. We’re really concerned because of Zika, but we need to rule out other causes of congenital infection to really make sure.

Microcephaly has received the most attention, but Ko found other problems with the infants.

“We’re seeing a spectrum. Many have fairly severe central nervous system lesions,” he explained. “There are also a lot of calcium deposits … Those can cause seizures and cause impairment in terms of function for the brain.”

He added, “We’re also seeing that in some babies, the brains, which usually have wrinkles, are actually smooth. That’s a sign that development of the brain has been impaired. Several of them are also impaired with respect to vision and hearing.”