This post has been updated with news that Obama is requesting $1.8 billion from Congress to fight the virus. 

Health officials say the Zika virus is "spreading explosively" across the Americas, and some think it seems destined to soon start spreading rapidly in the United States, too.

Already, three people in Washington, D.C., have the virus, including a pregnant woman. All three contracted it while traveling in Latin American countries, though officials say it is contained. In Dallas, a person contracted the virus after having sex with someone who had traveled from Venezuela. And Florida's governor has declared a state of emergency in five counties to prepare for a possible outbreak.

In all, nearly three dozen cases have been detected in 11 states and the District, and health officials say that number is growing.

U.S. health officials also say that some 200 million people live in areas that, come summer, might be conducive to the spread of the virus. The Washington Post's Lena Sun and Brady Dennis report that researchers are racing against time understand the virus — how it manifests itself and who's at risk.

And now, Congress is sounding the alarm as well.

Senate Democrats — all of them — sent a letter to President Obama on Friday morning urging the president to take a series of steps to try to stop the virus's spread. Several Senate Republicans have sent their own letter and Senate Republican leaders are scheduling committee hearings on the virus and briefings with top health officials.

"Given the public concern that followed the first Ebola case in our country," Senate Majority Leader Mitch McConnell (R-Ky.) said on the Senate floor last week announcing a briefing with Health and Human Services Director Sylvia Matthews Burwell, "I think we could all benefit from having a better understanding of what preparations are being made to protect Americans.”

But what exactly can be done, politically speaking? And why are Democrats asking Obama to do it rather than GOP leaders in Congress?

Congress will probably debate how much money it can give to the government to help fight a potential Zika outbreak in the United States. But it's up to the Obama administration to come up with a plan. Much of the action — coordinating with world health officials, researching the disease and a potential prevention drug, making guidelines on testing and how to keep people safe — falls in the hands of federal agencies, and those agencies fall under the purview of Obama.

On Monday, the White House announced Obama is asking Congress for $1.8 billion to help fight the disease abroad and stop it from spreading at home. (McConnell's spokesperson told The Fix in an email "Congress will review this new request" but indicated there are limited resources so Obama may not get everything he's asking for.)

But even with that extra cash, there's not a lot federal agencies can do right now beyond educating people on how to avoid it and researching ways to stop its spread. Here are three things they can do and are doing to try to stop Zika from spreading in the United States:

1. Tell people how to avoid it

This is the government's first — and probably most effective — line of defense. The Centers for Disease Control and Prevention has issued recommendations for Americans on how to avoid contracting Zika, and it's updating its recommendations regularly.

The Washington Post's Steven Mufson reports much of the money Obama wants to fight the virus would go to the CDC so it can continue to educate Americans on how to avoid getting it.

The CDC just issued their latest update Friday morning, which is bound to raise some eyebrows because it urges people to not have sex with pregnant women, period:

. Although mosquito bites are still the primary way to transfer the virus, there are reports in Dallas (and potentially elsewhere) of it spreading through sex. So the CDC has suggested people avoid sex with someone who is pregnant and definitely avoid sex with someone who has traveled to places where the Zika virus is already rapidly spreading.

The CDC also suggests you don't travel to places in the Americas you don't have to, especially if you're pregnant. It says be careful to avoid mosquito bites when you travel to the Caribbean, Central America, South America and the Pacific Islands — in all, 22 countries or territories and counting.

Senate Democrats also asked the Obama administration develop materials to distribute to travelers about how to avoid the virus.

2. Research ways to prevent it

There's more questions than answers right now about the Zika virus. Although it was first reported in a monkey in 1947, as of 2007 scientists knew of only 14 human cases.

Now there's more than 1 million in Brazil alone. The sudden rise of Zika helps explain why there is no drug to treat or prevent it yet, though researchers are feverishly working to produce something. Obama has asked Congress for $200 million to accelerate the search for a vaccine to protect Americans against the virus.

Members of Congress, from Senate Democrats to House Republicans, have urged Obama to "ramp up research efforts," including at the National Institutes of Health, to better understand the virus and its link to potentially brain-damaging and immune-system-damaging syndromes such as microcephaly and Guillain-Barré.

"We need to get ahead of this emerging threat," Rep. Vern Buchanan (R-Fla.) said in a press release urging quick congressional action and approving of Obama's attention the problem.

Right now, health officials are still trying to determine the exact link between Zika and birth defects. The CDC suggests testing newborns who are born with some of those diseases or who have mothers who have traveled to regions known to contain the virus. (There is no widely available test for this; it has to be done in specific government laboratories.)

3. Spray in places where the mosquito that carries Zika is spreading

A Houston-area county official suggested taking a page from Brazil, where authorities are going door-to-door to try to wipe out places where the mosquito known to carry Zika breeds.

Harris County, Tex., mosquito control director Mustapha Debboun told The Post's Sun and Dennis that if Zika were to spread there, workers would probably use handheld equipment to target-spray areas where mosquitoes breed.

Obama has requested $210 million to go to a fund to respond to outbreaks if (or some health officials say when) the virus appears in the U.S.

That's if the spread of Zika comes to that. And despite the government's best efforts, health officials here in the United States certainly say it's possible.