As the mosquito-borne Zika virus continues to infect people in a relentless wave, there is a worldwide rush to learn more about the disease.
Although Zika infections are usually mild, when they strike pregnant women, it is possible for the infections to lead to a condition called microcephaly in the women's children, which brings severe, lifelong impairments. Scientists are now working on a vaccine, but it could take years until it is ready for use, and people will continue to get sick in the meantime.
Today (Jan. 28), World Health Organization (WHO) officials predicted that between 3 and 4 million people worldwide would contract Zika virus, although they did not give a time frame for those cases, according to Scientific American. Meanwhile, U.S. officials announced that the United States will "likely" face small outbreaks of the virus, although larger outbreaks are unlikely.
Here's a look at the most commonly asked questions about Zika virus, and the science behind the answers. [7 Devastating Infectious Diseases]
Where did Zika originate?
Researchers first identified Zika virus in rhesus monkeys in Uganda in 1947, during a surveillance project to find yellow fever. A few years later, in 1952, they found the first human cases in Uganda and the United Republic of Tanzania, according to the WHO.
Since then, Zika outbreaks have occurred in Africa, the Americas, Asia and the Pacific region, the WHO said. Brazil experienced its first outbreak in May 2015.
How did Zika get to South America?
Zika is spread by mosquitos that belong to the genus Aedes, a group that lives on almost every continent. About 80 percent of people infected with Zika virus do not have symptoms, and they can unknowingly spread the virus to other Aedes mosquitoes. This includes people picking up the virus from mosquitoes in one region of the world, traveling, and then spreading it to mosquitos in another region.
Because of such travel, "now, you have the disease being established in new geographic areas," said Dr. Amesh Adalja, an infectious disease specialist and a senior associate at the University of Pittsburgh Medical Center's Center for Health Security.
How do people get Zika?
The primary way that people contract Zika virus is from the bites of infected mosquitoes.
Zika may also spread during sexual intercourse, although there are few confirmed cases of this occurring. In one case report, doctors described finding the virus in the sperm of a man in Tahiti in December 2013.
It's also possible that Zika could spread through blood transfusions, Adalja said.
There are no instances, to date, of breast milk transmission, and women in Zika areas should continue breastfeeding, he said.
What symptoms might infected people have?
It's unclear how soon people begin showing symptoms after they are exposed to the virus, but it's likely a few days, according to the WHO. But only about 1 in 5 people with the virus show any symptoms.
Those who become ill may have a fever, skin rash, conjunctivitis (pinkeye), muscle and joint pain, general discomfort and headaches, the WHO says. Symptoms typically last between two and seven days.
There is no treatment, but people who are sick with Zika virus can receive supportive care, such as pain relievers or fluids, if needed, Adalja said.
There are some reports linking Zika to an increasing incidence of Guillain-Barré syndrome, he said. This syndrome is an autoimmune disorder in which nerve cells are attacked, which causes weakness. The syndrome is sometimes incited by an infection. But the link is not clear, and researchers are looking into it. [The 9 Deadliest Viruses on Earth]
How might Zika virus affect pregnancies?
If a pregnant woman is infected with Zika virus, there is a chance that she could pass the virus on to her fetus. It's thought that Zika may be wholly or partially responsible for the sudden spike of microcephaly and other disorders seen in some Brazilian newborns.
However, there is no direct evidence that Zika causes these disorders.
"This is more of an association," Adalja told Live Science.
"The hypothesis is that when this virus is in the pregnant woman's blood, it crosses the placenta and infects the fetus in utero," he said. "Then, it has a negative effect on the brain development of the fetus, causing the fetus to be born with an abnormally small head, which is called microcephaly."
Microcephaly, "a devastating diagnosis," often leads to shortened life span and multiple developmental and intellectual disabilities, Adalja said.
Zika may also lead to other central nervous system challenges, such as problems with vision and hearing, he said.
Can a fetus become infected throughout the entire pregnancy?
"The greatest risk [of microcephaly occurring] is in the first trimester of pregnancy [when crucial brain development happens], but pregnant women in all trimesters should avoid travel to Zika areas if possible," Adalja said.
How are people tested for Zika?
Currently, people in the United States who have traveled to a region where Zika is being transmitted and who have symptoms of the infection are being tested by the Centers for Disease Control and Prevention (CDC) in coordination with various state health department labs, Adalja said. However, as time goes on, local health departments will likely start testing, and commercial tests will probably become available, he said.
To get tested, people can give blood or tissue samples to researchers, who can then test the specimen for genetic material from the virus. Researchers can also look for antibodies that the body has produced against the virus, but some of the antibody tests are not specific – they may give a positive test result if a person has antibodies to similar mosquito-spread viruses, such as yellow fever, Chikungunya and dengue.
If a pregnant woman is concerned that her fetus might be affected by Zika, she can get an ultrasound or an amniocentesis. The CDC published this flowchart to help pregnant women determine when and how they can get tested.
If Zika is linked to birth defects, why aren’t more children in Africa and Southeast Asia affected?
The microcephaly seen in some Brazilian babies is likely multifactorial, and could be caused by a whole slew of factors in addition to Zika, such as malnutrition, Adalja said. [7 Ways Pregnant Women Affect Babies]
However, there is also another hypothesis about why there aren't large numbers of children with microcephaly in regions that regularly have Zika infections: Perhaps people in these places got the virus when they were young, and developed antibodies against it, becoming immune to reinfection. It's possible that this immunity in pregnant mothers would protect fetuses, Adalja said.
If a woman gets Zika and then becomes pregnant a few months later, can the virus still affect the fetus?
No, there's no evidence that the virus persists after the acute stage of the illness is gone, Adalja said.
How far along are the vaccine tests?
Vaccines can take several years to develop as they go from concept to needle, Adalja said.
In the meantime, people can protect themselves by taking precautions against mosquitoes. (Wear bug spray and long clothing, avoid going out when mosquitos are active, get rid of standing water where mosquitos lay eggs, use mosquito nets or window screens and stay inside with air conditioning, as mosquitoes prefer humid environments).
The CDC also cautions that pregnant women should postpone travel to places infected with Zika virus. Detailed travel warnings can be found here.
Live Science newsletter
Stay up to date on the latest science news by signing up for our Essentials newsletter.
Laura is the archaeology and Life's Little Mysteries editor at Live Science. She also reports on general science, including paleontology. Her work has appeared in The New York Times, Scholastic, Popular Science and Spectrum, a site on autism research. She has won multiple awards from the Society of Professional Journalists and the Washington Newspaper Publishers Association for her reporting at a weekly newspaper near Seattle. Laura holds a bachelor's degree in English literature and psychology from Washington University in St. Louis and a master's degree in science writing from NYU.