Moving from Africa to French Polynesia to Central and South America, the disease is now active in the United States and Asia, and is likely to continue to spread.

Zika arrives in a country via the bloodstreams of globe-trotting vacationers and businesspeople, all returning home from areas where Zika is actively circulating. It's not like you can stop it: Four out of five people with Zika have no symptoms, so most of those passengers are unaware they carry the virus in their blood.

No country is immune from a potential outbreak, as long as the mosquitoes capable of spreading the virus -- Aedes aegypti and Aedes albopictus -- live there.

To make matters worse, Zika can also be transmitted by blood transfusions and through sexual contact, particularly oral, anal and vaginal sex, and even by secretions left on sex toys.

Some have called Zika the world's newest STD.

Why it's serious

Scientists have learned a lot about Zika in the past year. For one, the virus appears to have mutated as it has spread around the world.

When it first emerged, inconspicuously, from Uganda's Zika forest in 1947, the effects of infection were mild, nothing worse than a mild cold or flu. Though that's still true for many today, for others, the consequences of Zika are devastating. Babies are being born with life-altering brain damage to women infected with Zika during their pregnancies, immune-compromised adults are dying of complications, and a Zika-triggered auto-immune disorder called Guillain-Barré attacks the nervous system, causing temporary paralysis and even death.

Several countries in Central and South America are asking female residents to delay pregnancy if they can, and the Centers for Disease Control in the United States has told American women to stay away from areas where Zika is active . CDC sexual guidance is strict as well: no intercourse during the entire pregnancy unless the couple can carefully and correctly practice safe sex. Similar recommendations apply to couples who are trying to conceive.

"If you're pregnant, you're in a very delicate situation," said medical epidemiologist Dr. John Brooks, a senior medical adviser at the CDC's National Center for HIV/AIDS and STD Prevention . "You want to do everything you can to protect that baby. So our recommendation is to defer unprotected sexual contact for the entire pregnancy to protect against an outcome that could last a lifetime."

Vaccine on the way?

No vaccine currently exists. Even though several private and governmental agencies have versions in the works, the likelihood of having a vaccine ready for public consumption is a year or two away -- possibly even more.

"How soon, you know, depends on a couple of factors," National Institute of Allergy and Infectious Diseases Director Dr. Anthony Fauci said at a briefing about his organization's current vaccine , now starting phase one trials. "How effective is it? If it's a home run, we'll know pretty quickly. For longer-term issues, such as subtle safety issues that are really rare, we will need a larger trial."

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How long those trials may take, says Fauci, is determined by the number of Zika infections in a community when the vaccine is ready.

"If it's explosive in an area, then you can get an answer within 1½ years," Fauci adds, "but if the cases slow down, it might take us additional three to four years to determine if it works."

With a vaccine only a promise on the horizon, researchers are also looking at ways to treat the virus while it's circulating in the blood.

A number of groups are screening thousands of drugs that were already approved for human use by the Food and Drug Administration to see whether any might be effective against the Zika virus.

Researchers at Florida State University, for example, just announced that a drug approved to treat tapeworms also appears to block the Zika virus from replicating in Petri dishes in the lab. It's especially exciting, says study author Hengli Tang, because the drug is FDA-approved as a pregnancy category B drug, which means animal reproduction studies have yet to find a risk to a developing fetus.

"Because [it] is blocking the multiplication of the viral genetic material, the compound is equally effective even when added several hours after the cells are exposed to the virus," Tang added.

Fighting mosquitoes where they live

While scientists scramble to vaccinate and treat, mosquito control officials are doing their utmost to tackle the virus at its source: the mosquitoes themselves.

Traditional control techniques emphasize removing standing water, spraying pesticides and larvicides, and encouraging homeowners to keep screens on their doors and windows. That, along with the widespread use of air-conditioning, makes it much easier for developed countries like the United States to keep outbreaks contained. It's much harder in more economically disadvantaged countries.

Another factor straining traditional control efforts is the female Aedes aegypti mosquito, which is primarily responsible for this global spread. A crafty creature, she is an aggressive daytime biter who prefers to live close to her favorite blood meal, humans. Over the centuries, she has adapted to urban environments, preferring stagnant water and small containers such as plastic cups, potted plants and discarded tires to lay her eggs.

A "sip feeder," this bloodsucker likes to bite multiple people, taking just a sip of blood each time, increasing the number of people she can infect in just one feeding. The tiny creature's bite is almost painless.

Once you add that to her tendency to sneak up on victims, often biting on ankles and the back of elbows, you can see how easy it might be for her to feed without a life-threatening slap.

, The Zika-infected female can lay an average of 150 eggs at once, scattering them among several sites to improve survival. If dry, the eggs can survive up to a year says the World Health Organization , but once the wet season arrives, they immediately hatch. Studies show that some of those emerging babies carry the Zika virus, passed on from their mother, making them an immediate threat to any humans in their vicinity.

That's the worry for Brazil and other South and Central American countries, currently experiencing a drop in Zika cases during the drier winter months. When the rains begin again this fall, those eggs will hatch, and cases could begin to rise, along with the associated risk to pregnant women and their undeveloped fetuses.

Zika's long march

In the meantime, Zika continues its travels, hitch hiking in travelers' blood and via the occasional mosquito hidden in an industrial shipment of goods.

Whether the virus will take hold in a country depends on the local environment and a country's ability to combat the threat.

United States and Caribbean

The first case of locally transmitted Zika in the US was a surprise. Instead of occurring from a mosquito bite, the virus was sexually transmitted by a man recently returned from a country with an active outbreak.

Lots of travel-related cases followed; in fact, nearly every state in the union has reported a case of travel-associated Zika in their backyard. That has resulted in a total of 21 cases of microcephaly in the United States, a devastating birth defect in which a baby is born with a small head and brain, often leading to life-long neurological issues, even death.

In mid-July, a Miami mosquito bit a returning traveler, processed the virus and began spreading it in Wynwood, a 1-square-mile Miami neighborhood. Despite massive control efforts, one or two cases also popped up in Miami Beach and in the Tampa/St. Petersburg area.

To date, there are 43 cases of locally transmitted Zika in Florida.

That number pales in comparison with hard-hit Puerto Rico, which has over 13,000 cases of the virus, with more than a thousand of those pregnant women. An economic downturn, a failing health care system and inadequate financial support from the mainland makes it difficult for Puerto Rico to fight its epidemic.

Other Caribbean countries also continue to battle Zika as their outbreaks continue. The World Health Organization says Martinique has 10 cases of microcephaly, while just this week, the first babies with microcephaly were reported in Haiti, which has one, and the Dominican Republic with three cases.

Central and South America

Brazil continues to be the country hardest-hit by Zika, with thousands infected and over 1,800 babies diagnosed with microcephaly. That number doesn't count the babies with normal-size heads but who might develop vision, hearing and developmental issues from their Zika exposure.

Other Central and South American countries are also struggling with the fallout from the virus.

Columbia reports 29 cases of microcephaly in newborns, Panama has five cases, El Salvador four, French Guiana three and Paraguay two, while Honduras and Costa Rica report one case each.

The good news for South America is that colder winter weather and dry skies have reduced the number of new Zika cases. In fact, the WHO announced no laboratory-confirmed cases of Zika related to the Olympics, which ended August 21 in Brazil, and Chile hasn't reported a new case of Zika this year.

Europe

Europe has been spared much of Zika's devastation. Despite a number of imported cases from returning travelers, only Slovenia and Spain report cases of microcephaly -- one each -- the WHO says.

Asia

Southeast Asia has been hard-hit by Zika, and officials worry that numbers are vastly under-reported. Singapore just announced 82 cases of locally acquired Zika infection, with the number rising, but the WHO also lists Indonesia, Thailand, the Philippines and Vietnam as "countries with possible endemic transmission or evidence of local mosquito-borne Zika infections in 2016."

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Cambodia has reported cases in the past, but the WHO has it on a list of countries where the outbreak has either been "terminated" or is "without documentation" in 2016. Other countries that fit that positive category include French Polynesia, the site of a large outbreak in 2013; Laos; Malaysia; the Cook Islands; the Maldives; New Caledonia; Papua New Guinea; the Solomon Islands and Vanuatu.

Africa

Cape Verde, a small island 350 miles off the western coast of Africa, is reporting nine cases of microcephaly. Other than that, Africa, the birthplace of Zika, is not reporting any outbreaks at this time.

Worldwide

CNN has compiled the full list of countries and territories with current outbreaks of Zika, according to the CDC and the WHO.

Those affected are: Anguilla, Antigua and Barbuda, Argentina, Aruba, Bahamas, Barbados, Belize, Bolivia, Bonaire, Brazil, the Cayman Islands, Colombia, Costa Rica, Cuba, Curacao, Dominica, the Dominican Republic, Ecuador, El Salvador, Fiji, French Guiana, Grenada, Guadeloupe, Guatemala, Guyana, Haiti, Honduras, Jamaica, Kosrae (an Island of the Federated States of Micronesia), the Maldives, Marshall Islands, Martinique, Mexico, New Caledonia, Nicaragua, Panama, Papua New Guinea, Paraguay, Peru, Puerto Rico, Saba, Saint Barthelemy, Saint Lucia, Saint Martin/Sint Maarten, Saint Vincent and the Grenadines, Sint Eustatius, Suriname, Trinidad and Tobago, Turks and Caicos, United States of America*, the US Virgin Islands, Venezuela, Vietnam, American Samoa, Samoa, Tonga and Cape Verde.