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Three years after Zika virus first made headlines, and the photos of babies born with disturbingly small heads instilled fear in expectant parents around the world, doctors have learned a great deal about the disease. But while some of Zika's mysteries have been solved, other questions linger.

Between 2015 and 2016, thousands of babies whose mothers were infected with Zika during pregnancy were born with severe birth defects and a range of disabilities. Recife, Brazil was ground zero. There, Zika spread explosively in densely populated neighborhoods whose poor living conditions were a perfect breeding ground for the mosquitoes that spread the disease.

Then, as mysteriously as it arrived, the outbreak mostly subsided — from over 20,000 cases a week to just a few hundred.

Now, three years later, the children of the epidemic — they've been dubbed "Generation Zika"— are toddlers. Their families' lives have been turned upside down trying to secure the care and services these children need. Doctors struggle to provide treatment while at the same time keeping close watch on pregnant women for any sign of another outbreak.

"Every day that we talk to a mother planning a pregnancy, we can sense a concern. And this is a haunting menace," said Dr. Esper Kallas, an infectious disease specialist at the University of São Paulo.

Here's a look at what we know — and what we still don't fully understand — about Zika.

Where has Zika virus spread?

Zika emerged as a serious health concern in northeast Brazil in 2015. By 2016, 61 countries had experienced mosquito-borne Zika infections, according to the World Health Organization. Forty-seven of these countries had never seen the virus before. It spread through Latin America, the Caribbean, and as far north as South Florida and Texas.

Over 200 people contracted it in the United States, and even more Americans got infected while traveling abroad. The Centers for Disease Control and Prevention says there were no locally-transmitted cases in the U.S. in 2018 or so far in 2019.

Is it safe to travel?

Check the CDC's travel information page for up-to-date information about areas with a risk of Zika transmission. Health officials recommend pregnant women not travel to places where outbreaks are happening, and carefully consider whether travel is necessary to areas with possible risk.

"The large outbreak in the Americas is over, but Zika is and will continue to be a potential risk in many countries," the CDC says.

How does Zika spread?

Zika is primarily transmitted by Aedus aegypti mosquitoes. When one of these mosquitoes bites an infected person, it can pick up the virus and then spread it others when it bites them. The mosquitoes can bite four or five people in one blood meal, meaning it has the potential to spread the virus rapidly.

The insect thrives in warm climates and standing water. Recife's poorer communities were hit hardest by Zika because mosquitoes multiplied around homes without proper plumbing and moved freely through open windows with no screens.

Less commonly, Zika can be sexually transmitted by both men and women to their partners. Research is underway to better understand how long the virus stays in the semen and vaginal fluids of people who have Zika, and how long it can be passed to sex partners. It is known that Zika remains in semen longer than any other bodily fluids. The CDC recommends avoiding sex or using condoms for three months after possible Zika infection.

Even rarer, it's been documented that Zika can spread through blood transfusions, organ transplants, exposure in a laboratory setting, and from mother to baby during pregnancy, delivery, or breast-feeding.

What are the symptoms of Zika?

About four out of five people who are infected with Zika do not experience any symptoms at all and probably don't even know they have it. If symptoms do arise, they normally occur within about 14 days of exposure, and may include:

Mild fever

Skin rash

Muscle or joint pain

Conjunctivitis

In rare cases, people may develop an immune system complication called Guillain-Barré syndrome, which can cause at least temporary paralysis.

Most people fully recover from Zika. The greatest concern is for women infected early in pregnancy, because of the risks to the fetus.

What are the risks for pregnant women (or those who may want kids in the future)?

Even when a pregnant woman shows no symptoms of illness from the Zika virus, it can be passed to her fetus. Infection during pregnancy can lead to an array of health problems for the baby, including a serious birth defect called microcephaly (more on that below).

Not every pregnant woman with Zika will have a baby who's affected. Doctors aren't certain how likely that outcome is, but one study found that of 250 pregnant women in the U.S. with a confirmed Zika infection, 1 in 10 had a baby with virus-related birth defects.

The CDC reports the risk of birth defects appears to be greatest if Zika infection occurs during the first trimester.

The CDC recommends women avoid pregnancy for two months after possible exposure to Zika or after traveling to areas where it's spreading; they should wait three months if their male partner was exposed. But based on available evidence, researchers believe that a Zika infection in a woman who is not pregnant would not pose risks for her future pregnancies. And once someone has been infected with Zika, he or she is likely protected from a future infection with the same virus.

What is microcephaly?

In simple terms, microcephaly is a condition where a baby's head is much smaller than expected — less than the 3rd percentile, or smaller than 97 out of 100 people. The smaller the head and the earlier the brain was impacted during fetal development, the more serious the condition is.

Microcephaly can lead to seizures, developmental delay, intellectual disability, problems with movement and balance, feeding problems, hearing loss, and vision problems.

The Zika epidemic was the first time in history that a virus transmitted by a mosquito bite has been found to cause birth defects.

Though rare, microcephaly can also be caused by other viruses, alcohol, drugs or toxic chemicals during pregnancy, or if there's a disruption of the blood supply to the baby's brain during development.

What is congenital Zika syndrome?

Congenital Zika syndrome, or CZS, is the name that's been given to the pattern of birth defects found among children whose mothers were infected with Zika virus during pregnancy. These problems include microcephaly as well as issues affecting the baby's brain development, skull, eyes joints, and muscles.

Babies with CZS also experience challenges with sitting and standing on their own, feeding, and sleeping. Seizures and hearing and vision problems are also common.

Children born at the height of the Zika epidemic are now 3 years old and struggling with multiple health issues. Many, like Gleyse da Silva's daughter Gigi, still cannot walk or talk, and it's unclear if they ever will.

"There are still many uncertainties," da Silva said. "They don't have many answers. Even today there are a lot of questions."

Is there a treatment or cure for microcephaly or CZS?

There is no cure for microcephaly. Because symptoms range from mild to severe, a child's treatment options vary widely and depend on their individual case.

In Brazil, after the surge in cases, physical therapist Dr. Pepita Duran developed a personalized approach to treatment with exercises to help children improve motor function, a zero-gravity suit to improve posture, and aqua therapy to ease restricted muscles.

"In these three years, we have started to understand how each child is different," Duran told CBS News. "We find a unique approach for each child. The way we touch them, or engage them with a song or a cuddle, is different for each one."

Dr. Pepita Duran provides physiotherapy for Joao Guilherme, a 1-year-old with microcephaly, in Recife, Brazil, in 2016. Mario Tama / Getty Images

But of the hundreds of children diagnosed in the state, she can only offer free services to nine patients at a time. Other families have to rely on overcrowded public hospitals where wait times for appointments can take hours.

Is there a vaccine for Zika virus?

There is currently no vaccine for Zika, though scientists around the world are working hard to develop one.

In the meantime, the best way to prevent Zika is to protect yourself from mosquito bites. Check the CDC website for information about traveling to areas with risk of Zika and the precautions you can take.

Is there a treatment for Zika virus?

There is no specific treatment for Zika. Doctors recommend getting plenty of rest, drinking fluids to prevent dehydration, and taking acetaminophen to reduce fever. Aspirin and other non-steroidal anti-inflammatory drugs (NSAIDS) should not be taken until dengue can be ruled out to reduce the risk of bleeding.

People who are infected with Zika should take steps to protect others, including wearing insect repellent and using condoms during sex.

Will there be another Zika outbreak?

After Zika flared up in Recife, Brazil three years ago, the number of cases in the region just as quickly plummeted back down to very low levels. Dr. Carlos Reinaldo believes there is a simple explanation.

"There was no immunity against Zika" back in 2015, said Reinaldo, who is an obstetrician. "And practically the entire population was contaminated." Because of this, it created a herd immunity, and the outbreak then dwindled.

But while fears may have subsided, there are still some new cases in the region, a sign that not everyone is immune. In other densely populated areas of Brazil, less than 5 percent of people were infected with Zika, leaving millions more people potentially vulnerable to the virus in the future.

Infectious disease specialist Dr. Esper Kallas says it's a matter of when, not if, Zika will spread to other areas of Brazil like São Paulo.

"One day this virus is going to knock on our door," he said. "And it's going be transmitted."

Kallas is one of the doctors currently working on a vaccine in an effort to be prepared.

"If this happens again, we're not going to have time," he said. "We need to get it ready. We need preparedness."