With Zika moving through nearly every country in the Western Hemisphere over the past year and a half, it was only a matter of time before the virus made its way to the continental US.
At the end of July, health officials announced that a handful of Zika infections had originated from mosquitoes in Miami, Florida. As of August 22, there were 37 locally-acquired cases in Florida, mostly concentrated in a one square mile area of Miami's Wynwood neighborhood.
Zika was discovered in the 1940s, but most people had never heard of it until last year. That's because for decades, Zika outbreaks were sporadic and tiny, and the disease seemed to do little harm.
That changed in 2015. A massive outbreak in Brazil altered the scientific community's view of the mosquito-borne virus. Scientists learned that Zika can actually be a lot more dangerous than anyone thought, causing devastating birth defects in fetuses and even miscarriages. In light of that, the World Health Organization declared a public health emergency on February 1 this year.
Meanwhile, the virus has been on the move. To date, more than 60 countries and territories have reported mosquito transmission of the virus, and 11 countries have seen sexual transmission of Zika. The US is only the latest country to grapple with a Zika virus outbreak.
1) Zika is spreading locally in Florida — but don't expect it to go very far
Here's a quick recap of what's going on in Florida, the only state so far with native Zika transmission: On July 19, health officials announced that a woman living in Miami-Dade County, Florida, had tested positive for the mosquito-borne and sexually transmitted virus. But unlike all of the other Zika cases reported in the US at the time, she had no travel history to a country with Zika.
Since then, a small number of people in the area seem to have been similarly infected by local mosquitoes. The 37 cases are mostly concentrated in a one square mile area of Miami's Wynwood neighborhood.
But the virus is spreading. On August 19, health officials confirmed that five new cases had sprung up in the Miami Beach area — outside the one square mile in Wynwood. Another case has since popped up near Tampa, on the other side of Florida.
In response, the CDC expanded their travel advisory, warning pregnant women to avoid the part of Miami Beach where Zika is spreading, as well as the Wynwood area. They've also advised pregnant women and their partners to consider avoiding nonessential travel to Miami-Dade. According to the New York Times, this is likely to "threaten tourism in South Florida and deepen fears among pregnant women living in the area."
A small Zika outbreak in Florida wouldn't exactly be surprising. Zika has been identified in mosquitoes in more than 60 countries and territories over the past year, and the CDC has warned that the southeastern part of the US — particularly Florida and Texas — was at risk of outbreaks during mosquito season this year. It’s the region where the Zika-carrying Aedes aegypti mosquitoes thrive in the greatest concentrations.
But don't panic. Officials expect that Zika will likely follow the same pattern in the continental US as dengue fever, leading to small outbreaks in the southeastern US.
"Better housing construction, regular use of air conditioning, use of window screens and door screens and state and local mosquito control efforts helped to eliminate [mosquito-borne infections like malaria] from the mainland," said Lyle Petersen, director of the CDC's division of vector-borne diseases, in a statement this year. All these factors are expected to prevent widespread transmission here.
2) About 80 percent of people don't experience any symptoms with Zika
The vast majority of people who get Zika won’t even know they have it. A minority — roughly 20 percent — show relatively minor symptoms: a low-grade fever, sore body, and headache, as well as red eyes and a body rash. More rarely, this might include abdominal pain, nausea, and diarrhea. These symptoms usually appear two to 12 days after a bite and go away within a week, and with the same type of care as the flu — bed rest, fluids, etc.
There’s no vaccine or cure for the virus. But severe disease requiring hospitalization is uncommon, as is death from the virus. Once a person has been infected with Zika, they are thought to develop lifelong immunity.
But that's not the whole story. In rare occasions, Zika causes really serious health complications.
In both the Brazil and French Polynesia outbreaks, researchers noted that some people infected with the virus were later diagnosed with Guillain-Barré, a rare and sometimes deadly neurological condition in which people's immune systems damage their nerve cells, leading to muscle weakness and even paralysis. The symptoms can last weeks, months, or even years.
Again, though, this is rare: about one in 4,000 Zika cases, according to the WHO. And it’s not unique to Zika. Other viral diseases — like dengue and chikungunya — also cause Guillain-Barré.
More worryingly, Zika seems to have a particular appetite for the neurological cells of fetuses. When a pregnant woman catches the virus, her fetus faces a risk of severe birth defects, including microcephaly, which is characterized by a shrunken head and incomplete brain development.
The association between microcephaly and Zika has been seen in this outbreak in Brazil, as well as in reanalyses of the data from previous outbreaks, such as one in the French Polynesian islands in 2013 to 2014.
3) One percent of pregnant women who get Zika will have birth complications
The risk of birth complications is low. The WHO estimates that the fetuses of one in 100 women with Zika will go on to develop microcephaly or other abnormalities. And researchers believe the risks of Zika-related birth defects are greatest if a woman is infected in the first trimester of her pregnancy.
Zika isn’t the first virus to cause microcephaly. Rubella famously caused an epidemic of birth defects before the advent of the vaccine, and cytomegalovirus (CMV) can damage fetuses too.
But Zika is the first mosquito-borne virus to threaten fetuses. And even if these birth defects turn out to be a very rare complication of Zika, the risk to fetuses was enough to prompt the WHO to declare a rare global public health emergency earlier this year.
4) We don't fully understand all the ways Zika spreads
Zika is mainly carried by a specific type of mosquito called Aedes aegypti, which spreads the disease through bites.
What makes the Aedes aegypti a unique threat is that it is remarkably effective at carrying viruses — it's also the primary vector of the yellow fever, dengue, and chikungunya viruses.
Aedes mosquitoes are incredibly well-adapted to thrive alongside humans. They can breed and rest in small pools of water and moist environments around people's homes. (You can see examples of the mosquito's main aquatic habitats here, ranging from rain-filled cavities in trees to outdoor pots and animal drinking pans.) This is different from other types of mosquitoes, which prefer larger bodies of water.
Mosquito transmission, while thought to be the primary method, is not the only way to spread the virus.
As we’ve discussed already, Zika can also be passed from mother to child.
And it can be spread through sex. There have been more than 20 cases of Zika sexual transmission in 10 countries, involving oral, anal, and vaginal sex, and the virus has been shown to spread from women to men, men to men, and men to woman.
People can contract Zika through blood transfusions. In Brazil, there have been multiple reports of Zika spreading through blood transfusions, according to the CDC.
Finally, it's possible Zika can spread through saliva or urine, since live virus has been isolated from these bodily fluids, but there’s not a lot of clarity yet about this mode of transmission.
Researchers are also exploring other potential routes Zika spreads. In a mysterious recent case in Utah, a man who died from Zika passed it on to his caregiver, though the two didn’t engage in sex (the caregiver was reportedly the man’s son) and officials don't believe Aedes mosquitoes, which carry the virus, were involved, since they aren’t established in Utah.
5) Zika can be sexually transmitted — but simple precautions can reduce the risk
The virus stays in the blood for 11 days on average, and potentially much longer in other bodily fluids.
In men, the virus can persist in semen for more than 180 days (though it's unclear for how long it remains infectious). In women, researchers have found Zika in vaginal fluid 11 days after a woman showed symptoms of infection.
According to Margaret Honein, the CDC’s co-lead of the Pregnancy and Birth Defects Task Force for the Zika response, the latest reported time that a man was able to spread the virus through sex after a Zika illness was between 32 and 41 days.
6) Women exposed to Zika should wait at least eight weeks before becoming pregnant
First things first: If you’re pregnant, avoid mosquito bites, practice safe sex with your partner, and stay out of areas where the virus is spreading.
For couples who are thinking of becoming pregnant soon, the CDC’s recommendations for waiting to conceive vary depending on your likelihood of infection:
7) Men should avoid infecting a partner who may become pregnant
Again, the most important risk is really to pregnant women and their babies. So for men, the key concern is infecting a partner with Zika who may become pregnant. Either abstain from sex or use condoms for at least a period of eight weeks if you were in a place with Zika but showed no signs of infection, and up to six months if you had symptoms or were diagnosed with the virus (see the above chart for details).
8) If you're at risk of Zika, clean up your house, wear mosquito repellent, and practice safe sex
Since Zika is carried by mosquitoes and there's no vaccine, the best way to prevent outbreaks is to prevent mosquitoes that carry the virus from biting people.
The Aedes mosquitoes that carry Zika prefer small bodies of water around people's homes, so there are measures you can take to really limit mosquitoes' habitats, says Janet McAllister, a researcher in the division of vector-borne diseases at the CDC.
McAllister recommends "just generally keeping your yard clean and in shape, getting rid of debris or things stored outside that can hold water, or adjusting them so that they no longer hold water." She adds, "It's also important to maintain the vegetation in your yard — to keep the grass mowed and bushes trimmed, with good air flow."
Inside the home, the CDC recommends making sure you have screens on windows and doors that aren't punctured in any way. Wearing protective clothing, using insect repellents, and keeping your house cool with air conditioning also helps.
Since Zika can be spread through sexual contact, practicing safe sex (using condoms) with partners that may have come into contact with the virus is recommended, too. (See points 6 and 7.)
9) Before 2007, there were only 14 documented Zika cases
Zika is actually an old virus that only recently began to worry health experts. It was discovered in 1947 when it was isolated from monkeys in the Zika forest in Uganda. And for decades thereafter, it seemed to be barely bother humans.
Soon cases popped up in other Pacific islands, including a large outbreak in 2013-'14 in French Polynesia (388 cases). By May 2015, health officials had detected the virus in Brazil.
Zika has since spread to more than 60 countries and territories — mostly concentrated in Central and South America and the Caribbean.
All this means that we're in the throes of the biggest Zika outbreaks in history, and there's still a lot we have to learn about the virus.