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(CNN) A mosquito-borne virus is prompting worldwide concern because of an alarming connection to a neurological birth disorder and its rapid spread across the globe.

The Zika virus, transmitted by the aggressive Aedes aegypti mosquito, has spread to at least 34 countries and territories. WHO estimates 3 million to 4 million people across the Americas will be infected with the virus in the next year. The U.S. Centers for Disease Control and Prevention is warning pregnant women against travel to those areas; health officials in several of those countries are telling women to avoid pregnancy -- in some cases for up to two years.

"As long as the mosquito keeps reproducing, each and every one of us is losing the battle against the mosquito," Brazilian President Dilma Rousseff said recently. "We have to mobilize so we do not lose this battle."

The U.S. Defense Department is offering voluntary relocation to pregnant employees and their beneficiaries who are stationed in affected areas. Some airlines are letting their staff opt out of Zika infected routes

"That's a pandemic in progress," said Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases at the National Institutes of Health. "It isn't as if it's turning around and dying out; it's getting worse and worse as the days go by."

Here are five important things to know about Zika:

1. What is Zika and why is it so serious?

The Zika virus is a flavivirus, part of the same family as yellow fever, West Nile, chikungunya and dengue. But unlike some of those viruses, there is no vaccine to prevent Zika or medicine to treat the infection.

Zika is commanding attention because of an alarming connection between the virus and microcephaly , a neurological disorder that results in babies being born with abnormally small heads. It causes severe developmental issues and sometimes death.

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Since October, Brazil has seen 508 confirmed cases of microcephaly in newborns. At least seventeen of those cases have a confirmed link to the Zika virus. There were only 146 cases in 2014. So far, 27 babies have died from the condition, with at least five linked to Zika. An additional 70 deaths are under investigation, and authorities are investigating 3,935 suspected cases.

Other Latin American countries are seeing cases in newborns as well. Colombia reported more than 6,000 pregnant women have tested positive for the virus, while in the United States one Hawaiian baby was born with microcephaly linked to the Zika virus after his mother returned from Brazil.

Health officials are looking at a couple of first trimester miscarriages among women who had traveled to infected areas and became ill. They can't say with certainty whether Zika caused the pregnancy losses. More than 20 states have confirmed the virus in individuals who traveled to areas where the virus is circulating. Some states are following pregnant women with the virus.

The CDC is asking OB-GYNs to review fetal ultrasounds and do maternal testing for any pregnant woman who has traveled to one of more than 30 countries where Zika is active.

The agency advises health care providers offer testing for the virus to pregnant women who have traveled to those areas within two to 12 weeks after returning home.

Providers should screen pregnant women with symptoms of the virus while they are experiencing illness and should test pregnant women without symptoms at the start of their prenatal care and run a subsequent test in the middle of the second trimester, the CDC said. Providers may want to consider an additional ultrasound beyond the routine second trimester ultra, the agency said.

In most people, symptoms of the virus are mild, including fever, headache, rash and possible pink eye. In fact, 80% of those infected never know they have the disease. That's especially concerning for pregnant women, as this virus has now been shown to pass through amniotic fluid to the growing baby.

"What we now know," said Dr. Lyle Petersen, director of the CDC's Division of Vector-Borne Diseases, "is that fetuses can be infected with the virus. That's not new for infectious diseases, but it is new for this virus."

Fauci agreed, "This is a very remarkable and unusual situation, because the other viruses don't do that to our knowledge. You just don't see that with dengue or West Nile or chikungunya."

2. How is Zika spread?

The virus is most commonly transmitted when an Aedes mosquito bites a person with an active infection and then spreads the virus by biting others. Those people become carriers when they have symptoms.

In early February, the CDC reported the first case of locally acquired Zika virus in the United States in this outbreak , but it was not from a mosquito bite. Instead it was passed via sex. The individual had sex with a partner who had recently returned from Venezuela infected with the mosquito-borne virus. The patient had not traveled.

Now federal and state health officials are investigating more than a dozen cases of possible sexual transmission of the virus, some of them among pregnant women. This could mean sexual transmission is more likely than previously thought, according to the CDC.

"There have been isolated cases of spread through blood transfusion or sexual contact and that's not very surprising," CDC Director Tom Frieden told CNN's Dr. Sanjay Gupta. "The virus is in the blood for about a week. How long it would remain in the semen is something that needs to be studied, and we're working on that now."

Previously, there had been only three documented cases linking Zika to sex. During a 2013 outbreak in French Polynesia, semen and urine samples from a 44-year-old Tahitian man tested positive for Zika even when blood samples did not. A second documented case in the United Kingdom in 2014 found high levels of the Zika virus in semen up to 62 days after the onset of the illness; in fact the viral load was stronger at that time than when the first samples were taken.

Even earlier, in 2008, Colorado microbiologist Brian Foy contracted Zika after travel to Senegal ; his wife came down with the disease a few days later even though she had not left northern Colorado and was not exposed to any mosquitoes carrying the virus.

But the CDC noted: "There have been no reports of sexual transmission of Zika virus from infected women to their sex partners."

The CDC said there have been documented cases of virus transmission during labor, blood transfusion and laboratory exposure . While Zika has been found in breast milk, it's not yet confirmed it can be passed to a baby through nursing.

The FDA has taken measures to protect the U.S. blood supply by asking individuals who have returned from infected areas to defer donating blood for four weeks after their return. If they've had symptoms of the virus, they are asked to hold off and if they've had sexual contact with someone who has returned from travel. Canadian Blood Services and the Red Cross have made similar recommendations.

The CDC's Frieden said while research continues on sex and other avenues of transmission, the "vast majority of spread" of the Zika virus is going to be from mosquitoes. "The bottom line is mosquitoes are the real culprit here," he said.

3. Where is the Zika virus now?

The Zika virus is now being locally transmitted in Aruba, Barbados, Bolivia, Bonaire, Brazil, Colombia, Puerto Rico, Costa Rica, Curacao, Dominican Republic, Ecuador, El Salvador, French Guiana, Guadeloupe, Guatemala, Guyana, Haiti, Honduras, Jamaica, Marshall Islands, Martinique, Mexico, Nicaragua, Panama, Paraguay, Saint Martin, Suriname, Trinidad and Tobago, U.S. Virgin Islands, Venezuela, American Samoa, Samoa, Tonga and Cape Verde, according to the CDC and WHO.

Zika has arrived in the United States from travelers returning from these infected areas and, in one confirmed case and 14 suspected cases, through sexual transmission. The concern, of course, is whether imported cases could result in more locally transmitted cases within the United States.

The Aedes albopictus, or Asian tiger mosquito, which along with Aedes aegypti transmits Zika virus, is present in many parts of the United States.

If mosquitoes in the United States do become carriers, a model created by Toronto researchers found more than 63% of the U.S. population lives in areas where Zika virus might spread during seasonally warm months . A little more than 7% of Americans live in areas where the cold might not kill off the mosquito in the winter, leaving them vulnerable year-round.

4. What can you do to protect yourself against Zika?

With no treatment or vaccine available, the only protection against Zika is to avoid travel to areas with an active infestation. If you do travel to a country where Zika is present, the CDC advises strict adherence to mosquito protection measures: Use an EPA-approved repellent over sunscreen, wear long pants and long-sleeve shirts thick enough to block a mosquito bite, and sleep in air-conditioned, screened rooms, among others.

If you have Zika, you can keep from spreading it to others by avoiding mosquito bites during the first week of illness, the CDC said. The female Aedes aegypti, the primary carrier of Zika, is an aggressive biter, preferring daytime to dusk and indoors to outdoors. Keeping screens on windows and doors is critical to preventing entry to homes and hotel rooms.

For women of childbearing age, the CDC now recommends discussing, "strategies to prevent unintended (pregnancies), including counseling on family planning and the correct and consistent use of effective contraceptive methods in the context of the potential risks of Zika virus transmission" with patients.

The CDC also called on local health officials to implement routine testing recommendations for pregnant women with or without symptoms based on local transmission of the virus and their capacity to process them.

5. What's being done to stop Zika?

Researchers are hard at work in laboratories around the world trying to create a Zika vaccine . A clinical trial for a Zika virus vaccine could begin this year, according to Fauci. The WHO says it will be at least 18 months until large-scale clinical trials get underway.

"While in development, it's important to understand we won't have a vaccine this year or even in the next few years, although we may be able to have a clinical trial start this calendar year," he said.

Health officials are implementing traditional mosquito control techniques such as spraying pesticides and emptying standing water receptacles where mosquitoes breed. The CDC encourages homeowners, hotel owners and visitors to countries with Zika outbreaks also to eliminate any standing water they see, such as in outdoor buckets and flowerpots.

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Studies show local control is only marginally effective since it's hard to get to all possible breeding areas. And since Aedes aegypti has evolved to live near humans and "can replicate in flower vases and other tiny sources of water," said Foy, the Colorado microbiologist, the mosquitoes are difficult to find and eradicate.

Another prevention effort is OX513A, a genetically modified male Aedes aegypti, dubbed by critics as the "mutant mosquito" or "Robo-Frankenstein mosquito." The creation of British company Oxitec, OX513A is designed to stop the spread of Zika by passing along a gene that makes his offspring die. Since females only mate once, in theory this slows the growth of the population. Each OX513A carries a fluorescent marker, so scientists can track him.

Key West, Florida, residents gave the genetically modified male his monster nicknames while protesting a trial release of the mosquito in 2012 as a way to combat an outbreak of dengue fever in South Florida. The Food and Drug Administration is reviewing that effort.

But field trials in Brazil in 2011 were successful, according to Oxitec , eliminating up to 99% of the target population. A new release of males in the Pedra Branca area of Brazil in 2014 was 92% successful, the company said. The mosquito has also been tested in the Cayman Islands, Malaysia and Panama.