Officials at the U.S. Centers for Disease Control and Prevention (CDC) say there is now no doubt that the Zika virus causes rare fetal brain defects.

Dr. Thomas Frieden, director of the CDC, said new research published Wednesday afternoon in the New England Journal of Medicine “marks a turning point in the Zika outbreak.”

“It is now clear that the virus causes microcephaly,” he said in a press statement. “We are also launching further studies to determine whether children who have microcephaly born to mothers infected by the Zika virus is the tip of the iceberg of what we could see in damaging effects on the brain and other developmental problems.”

He added this is the first time a disease transmitted by a mosquito has been linked to a birth defect,

More than 1,100 cases of microcephaly have been confirmed in Brazil since the Zika outbreak began there last fall.

Scientists at the CDC have waited to officially make the connection until enough scientific evidence was in, Frieden added.

CDC officials were quick to point out that not all pregnant women who are infected with the Zika virus deliver babies with brain defects. It simply increases the risk.

The CDC continues to advise pregnant women to avoid traveling to areas where the Zika virus is spreading. Today the agency added Saint Lucia in the Caribbean to its list of countries with Zika-related travel notices.

In response to a question at a press briefing today, Dr. Sonja A. Rasmussen, the CDC’s director of the division of public health information and dissemination, said women who are infected with the Zika virus and get pregnant later should not be overly concerned.

She said the Zika virus has to be in a woman’s bloodstream to affect an unborn child. After a woman is clear of the virus, it is probably safe to become pregnant.

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Observing the Attack

Earlier this week, scientists in Brazil announced they might have figured out why the Zika virus damages the brains of unborn children.

The researchers said they have discovered that the Zika virus kills developing brain cells, a development that would explain why the disease has been linked to brain defects such as microcephaly.

Using stem cells in a laboratory, the team observed human neural stem cells derived from induced pluripotent stem cells grown in clusters. The system represents models of embryonic brain development in fetuses.

The researchers infected the growing cells with a Zika virus that had been isolated from a Brazilian patient.

Within days, the virus had killed most of the neural stem cells.

Separately, a cluster of neural stem cells that weren’t infected with the virus continued to grow, similar to that of a healthy developing fetus.

In a second experiment, the team used neural stem cells grown as brain organoids, or artificial cells meant to mimic a brain. They observed that the Zika virus reduced the organoid growth by 40 percent compared to a cluster that wasn’t infected.

In a third experiment, clusters of neural stem cells infected with the dengue virus fared better than the Zika-infected cells.

From that, the researchers concluded, specifically the Zika virus and not its entire viral family cause the brain damage.

Their findings were published in the journal Science.

Read More: Dengue Vaccine Could Pave the Way for Zika Vaccine »

Another Brain Disease

Officials also announced this week that the Zika virus might be linked to yet another brain disease.

The announcement from the American Academy of Neurology stated that the virus might be associated with an autoimmune disorder that attacks the brain’s myelin in a similar way to multiple sclerosis.

The academy said scientists at the Restoration Hospital in Brazil had examined 151 patients who came to the hospital between December 2014 and December 2015 with symptoms compatible with the virus family that includes Zika and dengue fever.

Six of the patients developed neurological symptoms consistent with autoimmune diseases. All six were eventually diagnosed with having the Zika virus.

Four of those patients had Guillain-Barré syndrome.

The other two developed acute disseminated encephalomyelitis (ADEM). This disease causes swelling of the brain and spinal cord. Unlike multiple sclerosis, however, ADEM usually consists of a single attack that most people recover from within six months.

Nonetheless, five of the six people with the autoimmune conditions still experienced motor functioning problems when they were released from the hospital. One person had vision problems and another had problems with memory and cognitive function.

“This doesn’t mean that all people infected with Zika will experience these brain problems. Of those who have nervous system problems, most do not have brain symptoms,” Dr. Maria Lucia Ferreira with Restoration Hospital said in a press release. “However, our study may shed light on possible lingering effects the virus may be associated with in the brain.”

Their findings will be presented Friday at the annual meeting of the neurology academy.

Most people with a Zika infection only suffer mild symptoms that last a week or so. In the United States, 350 Zika cases have been confirmed. All were people who had traveled to areas with Zika outbreaks.

Read More: Guillain-Barré Syndrome »