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How does Zika virus spread? What we know so far

The rapidly spreading Zika virus -- and its alarming link to birth defects -- has raised anxieties and prompted many questions about how it is transmitted and who's most at risk.

The primary concern over Zika is its suspected link to microcephaly, a condition in which a baby is born with an abnormally small head, which can lead to mental retardation and other complications.

The virus has also been linked to Guillain-Barré syndrome, a rare neurological illness that can cause paralysis.

But questions remain over how Zika is transmitted -- and not all the answers are entirely conclusive. Here's what we know so far about how the virus is spread.

Mosquito bites

There is substantial evidence that Zika virus is primarily spread to humans through the bite of an infected mosquito.

Mosquitoes: "The most murderous animal on earth"
Mosquitoes: "The most murderous animal on earth"

The Aedes aegypti mosquito -- the species that carries Zika -- "can bite four or five people in the course of one blood meal, meaning it can spread disease quickly," CDC Director Dr. Tom Frieden told a congressional committee earlier this week.

Zika virus has been reported in over two dozen countries, mostly in Latin America and the Caribbean.

So far in the continental U.S., state and local health officials report at least 79 travel-acquired cases of Zika virus -- meaning people who recently visited areas where Zika is actively spreading were infected there and came back home with the illness.

There have been zero cases of mosquitoes spreading Zika infection in the U.S. But as the mosquitoes become more prevalent in warmer weather this spring and summer, that may change.

The Aedes aegypti mosquito is primarily found in southern states, so as more Zika-infected travelers return home, mosquitoes here could bite them, pick up the virus, and transmit it to others.

99 million travelers departed from major Brazilian airports for international destinations from Sept. 2014 to Aug. 2015. More than 2.7 million of them traveled to the U.S. The circles on this map indicate areas with greater numbers of travelers from Brazil. Areas shaded in yellow have mosquitoes that could transmit Zika virus during warmer months, while areas in orange have them year-round. The Lancet/CBS News

However, while experts expect there to be some local transmission in the U.S., they do not anticipate any major outbreaks in this country.

"If Zika acts like the other types of viruses that are mosquito-borne that we've had experience with, like dengue and chikungunya, we will see mini-outbreaks like in Florida and in Texas that can be well controlled with mosquito vector control. Hopefully we will not see anything worse than that, but we have to be very vigilant," Dr. Anthony Fauci, Director of the U.S. National Institutes of Health's National Institute of Allergy and Infectious Diseases, told "CBS This Morning."

Several factors put the U.S. at an advantage to help keep Zika at bay, including less dense urban areas than South and Central America, access to air conditioning and protective screens, and overall better mosquito control.

Everyone can protect themselves against mosquito bites by wearing long-sleeved shirts and long pants, staying in places with air conditioning or that use window and door screens, and using Environmental Protection Agency (EPA)-registered insect repellents, such as those containing DEET, Picaridin or a combination.

CDC travel health experts recommend choosing a formula with at least 20 percent DEET.

Sexual transmission

There have been several instances of sexual transmission of Zika virus, including a case in Texas earlier this month.

According to Dallas County health officials, a patient there was infected with Zika after having sexual contact with an ill person who recently returned from Venezuela, where Zika virus is being spread by mosquitoes.

The CDC -- drawing on three known cases of sexual transmission of Zika in the medical research -- reports that the virus may be present in semen for up to 10 weeks after the onset of illness.

However, it is not known if Zika can persist in semen for longer than 10 weeks or if infected men who never develop symptoms can transmit Zika virus to their sex partners.

As such, the CDC recommends that men who live in or have travelled to a Zika-affected area and have a pregnant partner should abstain from sexual activity or consistently and correctly use condoms during vaginal, oral, and anal sex. (Health officials emphasize the importance of protecting pregnant women because of the risk Zika poses to unborn babies.)

Men who have been in Zika-affected areas should also consider abstinence or condom use during sexual contact with non-pregnant partners, the agency advised.

As more research is done on the risk of Zika through sexual transmission, the CDC is expected to update its guidelines.

Blood transfusions

Earlier this month, Brazilian health officials confirmed two cases of Zika transmission through blood transfusions from donors who carried the virus, Reuters reports.

Reporting last week from Recife, the Brazilian town at the epicenter of the Zika virus outbreak, CBS chief medical correspondent Dr. Jonathan LaPook noted that many experts think it's a matter of time before all Brazilian blood transfusions can be tested for the virus.

"We can start to do this, of course," researcher Rafael Franca told CBS News. "We have the machines, we have the technology, we have the people to do that. We need to make sure that the test is going to give solid results."

In the U.S., major blood banks, including the American Red Cross, have asked people not to donate blood if they've traveled in the last 28 days to Mexico, the Caribbean, South or Central America.

The Red Cross is also asking donors who give blood and subsequently develop symptoms of Zika virus within 14 days of the donation to notify the organization so their donated blood can be quarantined.

Other bodily fluids

Scientists have detected evidence that Zika virus can be found in breast milk, saliva, and urine samples. However, at this time there have been no reports of transmission in any of these ways, and the CDC has not issued any cautions about breast feeding or kissing.

"From what we know, and Dr. Anthony Fauci, who's the head of infectious diseases for the NIH told me, just because you have virus in the saliva doesn't mean that you can give it to somebody that way," LaPook said. "I think that's a very important thing."

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