The Centers for Disease Control and Prevention gathered more than 300 local, state, and federal authorities and experts at its Atlanta headquarters Friday to prepare for clusters of mosquito-transmitted Zika infections on the US mainland.

“The mosquitoes that carry Zika virus are already active in US territories, hundreds of travelers with Zika have already returned to the continental US, and we could well see clusters of Zika virus in the continental US in the coming months,” CDC Director Tom Frieden said in a statement prior to today’s meeting. “Urgent action is needed, especially to minimize the risk of exposure during pregnancy.”

Zika, a virus that has been tearing across Central and South America since last year, is mostly transmitted by mosquito, but it can also be spread through sexual contact. Generally the virus only causes mild illness, with symptoms including fever, rash, pink eye, and aches. But in the recent outbreaks, Zika has been linked to rare cases of paralyzing auto-immune disease, called Guillain-Barré syndrome. Of most concern, it's also linked to devastating birth defects, including microcephaly, in which babies are born with small, malformed heads and brains.

While researchers are still studying the link between Zika and microcephaly, health experts fear that microcephaly is just one of the potential problems for the unborn. “Perhaps one of the most important unknowns is what is the range of fetal abnormalities in addition to microcephaly,” Frieden said in a press conference during the summit. Microcephaly may just be the extreme, he and others noted. Babies exposed to the virus in utero may also suffer from less obvious developmental and cognitive problems, he speculated.

The fear is bolstered by recent data that has only strengthened the tie between the virus and the birth defect, with some studies finding the virus killing off developing brain cells. In a study released this week in the New England Journal of Medicine, researchers report tracking the development of a fetus whose mother was infected with the virus during a trip to Central America while she was three months pregnant.

With blood tests and magnetic resonance images (MRI), researchers watched as the baby’s brain essentially turned to liquid in the course of nine weeks. The woman aborted the fetus at week 21.

Friday’s one-day summit covered such breaking scientific data on the virus and provided training to authorities on how to prevent, treat, and talk with the public (particularly pregnant women) about Zika and its health effects. Experts also focused on coordinating efforts to stamp down mosquito populations.

There’s a hodge-podge of practices in various communities for tackling mosquito control, and many of them are very effective, according to Amy Pope, a White House deputy homeland security advisor and deputy assistant to the president who spoke at the press conference. “The goal of today’s summit is to bring all of those practices together in one place, give folks sort of the menu of options, so that they can develop a comprehensive plan well in advance of when we see mosquitoes biting around the continental United States,” she said.

Though health experts don’t foresee extensive mosquito-borne outbreaks of Zika in the US, there’s reason to expect small clusters of transmission. Zika is transmitted by Aedes mosquitoes, particularly Aedes aegypti and to a lesser extent Aedes albopictus. These mosquitoes, which are present in some areas of the US, can also transmit yellow fever, dengue, and chikungunya viruses. Small outbreaks of chikungunya and dengue pop up in certain areas each year, particularly in Texas and Florida. Health experts suspect that Zika may behave similarly.

Frieden stressed the difficulty of knocking back Aedes populations, which are day-biters that can breed in very small amounts of standing water. Coordinated, sustained, and well-funded efforts are needed to control these populations, he said.

So far, there is no vaccine or specific treatment for Zika. However, in another scientific report in the journal Science this week, researchers report getting the first detailed, 3D image of the virus using cryo-electron microscopy. While the viral close-up looks unsurprisingly similar to that of dengue—a related virus—there are minor differences. Those findings could provide clues to how researchers might defeat the virus with a vaccine.

New England Journal of Medicine, 2015. DOI: 10.1056/NEJMoa1601824 (About DOIs).

Science, 2015. DOI: 10.1126/science.aaf5316