Zika has been spreading throughout the Western Hemisphere, carried mainly by a type of mosquito called Aedes aegypti. The virus is a major concern in countries such as Brazil, Venezuela, and El Salvador, and mosquitos have begun to transmit the virus locally in Puerto Rico.

Zika isn't yet transmitting in mosquitoes the United States — all of the people with Zika here were infected while traveling or through sexual transmission. But the number of those cases is rising. According to a May 18 update from the Centers for Disease Control and Prevention, there are currently 544 Zika infections in the US. And as of May 12, the CDC reports there are 157 pregnant women with a possible Zika infection on the mainland.

So what are the chances that we will see a Zika outbreak in the United States this summer? That's not easy to answer, but researchers have some clues.

Zika isn't being transmitted by mosquitos in the United States mainland, yet

The map above shows where the virus is actively being transmitted from mosquitos to humans around the world.

In Central and South America, the World Health Organization estimates the virus has infected 3 to 4 million people, fueling an uptick in a birth defect called microcephaly. The CDC now believes there's a direct causal relationship between the virus and the malformation.

Health officials are worried the map will change, and Zika will come to infect the Aedes aegypti mosquitos we have here.

All it would take would be for the mosquitos to bite one of these infected travelers and suck the virus up into their bodies. The mosquitos are essentially syringes with wings, and can quickly infect thousands of people.

There's also a fear that the Asian tiger mosquito (Aedes Albopictus), a species of mosquito that has a much wider range in the United States than Aedes aegypti, could also spread the virus. If that were to happen, it could fuel a spiraling outbreak like the one that's occurring in Central and South America. For now, the aegypti variety is the main concern.

The true nightmare scenario in the US — a large epidemic of babies with microcephaly — is unlikely. Screened-in windows and air conditioning units are more common here than in infected countries, and they keep mosquitoes out of our homes. Plus, the state that is most at risk, Florida, has lots of experience dealing with outbreaks of mosquito-borne diseases like dengue fever and chikungunya.

But the CDC can't completely rule it out.

The best it can do is predict the potential spread of Zika by looking to past outbreaks of dengue and chikungunya, viruses that also spread via the aegypti mosquito. "We can't guarantee Zika to behave as those two other viruses behave," CDC director Tom Frieden said at a National Press Club event May 26.

But considering the mosquito is the same, he said, the CDC can make some educated guesses. He offered some predictions and laid out a few possible scenarios.

1) Travel-related Zika cases will continue to pop up all across the US.

2) In tropical US territories like Puerto Rico and the Virgin Islands, there will likely be hundreds of thousands of infections

3) If Zika arrives in Hawaii, it could spread slowly there.

4) In parts of Florida and Texas, there may be small clusters of outbreaks. In the past, dengue and chikungunya outbreaks in these areas "have not been widespread," Frieden said, because "local governments have been very effective at mosquito control to prevent widespread infection." But, he added, "We do expect there will be some spread — through mosquitos — in [these] parts of the continental US."

Frieden notes that the CDC's top priority is to protect pregnant women, who may show symptoms of the virus if they're infected. It's now believed that birth defects occur in between 1 and 13 percent of pregnant women with the virus.

Meanwhile, he stressed the CDC needs additional emergency funds from Congress to kick these prevention efforts into high gear.

Some US cities are more at risk than others

Frieden identified Florida, Texas, Puerto Rico, and Hawaii as potential ground zeros for Zika infections. But some other cities may also be at risk.

The National Center for Atmospheric Research produced the map above after analyzing the relative Zika risk of 50 US cities.

Its research, which was published in PLOS Current Outbreaks in March, combines meteorological records, simulation of Aedes aegypti mosquito population growth, estimates of human-mosquito exposure, and transportation data.

The researchers' aim is to look ahead at the summer months and help public health officials prepare for potential Zika outbreaks.

The new map shows the places where — if this were to happen — there would be a higher risk of outbreaks in the coming months. While the Aedes aegypti, the mosquito most likely to transmit the virus in the US, can't tolerate the cold of winter in many US cities, its numbers start to increase as temperatures rise. The exceptions are Florida and Texas, where the mosquitoes can live year-round.

It's not an absolute ranking. Rather, the map shows where the most critical factors align to cause an outbreak. Overall, the findings are pretty simple. Generally, cities with the highest levels of travel, the highest numbers of mosquitoes in peak summer months, and those that are in the climate range in which the mosquitoes thrive are the most at risk.

Here's a more detailed version of the map from the PLOS study. It's a bit harder to scan, but has some useful additional information.

(Note: The CDC's estimate of the potential range of the mosquitos is a bit larger than the one shown here.)

In deep red, it highlights areas where outbreaks of dengue and chikungunya — both spread by the aegypti mosquito — have occurred.

This version of the map also shows the extent of the Asian tiger mosquito, the bug that's believed to also transmit the disease, but hasn't yet figured into a major outbreak.

You can see on the map the constellation of risk factors all converge in Florida, which has already declared a state of emergency. The state is already revving up disease surveillance and mosquito control efforts.

The study does note that all the 50 cities could, in theory, have a climate suitable for the mosquitoes by mid-July. (But there's really no reason to fear they'd suddenly make a home in cities they historically haven't been found in.) But as a general rule, Southeastern cities — with their somewhat tropical climate and abundant mosquitoes — are the most at risk. The researchers also note there's a chance, due to El Niño, that this summer will be hotter than normal, fueling additional mosquito hatchings in the Southeast.

"Memorial Day weekend heralds the start of the mosquito season in the US," Frieden said at the Press Club. "We have a narrow window of opportunity to scale up Zika prevention measures, and that window of opportunity is closing."