Brazilian authorities have advised pregnant partygoers at Carnival, the riotous celebration that has come to symbolize Brazil, not to kiss strangers — just to make sure they don't get Zika.
The U.S. Centers for Disease Control and Prevention has added condoms to a long list of precautions to avert Zika infection. The list also includes DEET and long sleeves.
The news has people worrying and wondering about just how Zika spreads. Here's what scientists say they do and do not know:
How it spreads
It's mostly mosquitoes
"I think it's important to step back and emphasize that Zika is a mosquito-borne virus, and the overwhelming majority of cases are spread by mosquitoes," CDC director Dr. Thomas Frieden told reporters last week. "The Aedes aegypti is an aggressive mosquito. It bites four or five people at one blood meal," he added. "So everything that we've seen about not just Zika, but dengue and chikungunya suggests that this is the overwhelming cause of transmission."
There are two documented cases of Zika virus in a man's semen, and one documented and one suspected case of a man infecting someone else that way: once, in 2008, when a researcher returned from Africa and infected his wife, and more recently in Dallas, where officials say a traveler to Venezuela infected his sex partner.
"We don't know how long the Zika virus may persist in semen, but it will be weeks to months before we know more," Frieden said. "Right now, there are a total of three cases in the world literature of Zika virus being detected in male genital urinary secretions." Most health experts say they do not believe Zika is commonly spread this way.
Any virus can spread in blood, and Zika is no exception. "It goes away from the blood within about a week," Frieden said. "So we wouldn't be surprised to see, for example, occasional transfusion-associated cases if someone gave blood when the virus was in their blood.
That's why, he said, "the American Association of Blood Banks issued the guidance to defer blood donations for all individuals who had traveled to an area with Zika transmission for 28 days. That was a margin of safety for blood transfusion."
How it probably doesn't spread
Frieden is very careful not to say it's impossible that Zika could spread through saliva or kissing, but Dr. Anthony Fauci, head of the National institute of Allergy and Infectious Diseases, says it is unlikely.
The test used to detect Zika in saliva looks for genetic material, not a viable virus that can cause infection. And pieces of genetic material are unlikely to cause an infection. The human immunodeficiency virus (HIV) that causes AIDS is also found in saliva, but saliva has not been shown to transmit the virus. The exception is if there's blood in the saliva, but in that case it's the blood and not the saliva that's transmitting the virus.
Some viruses can be spread by kissing. The most notorious is herpes — but colds, flu and a few other viruses can be, too.
That's another unknown but again, scientists say it's unlikely. A deadly virus called hantavirus is spread through rodent urine, as are some other rodent-spread diseases. Most viruses are adapted to spread through a specific vector. Zika is spread by mosquitoes and virologists say it would be unlikely to also be transmissible through urine.
Some viruses do spread through the air. Measles is the most notorious, and flu can spread on tiny droplets that remain suspected in the air. But these viruses are very different from Zika, a flavivirus in the same family as dengue virus, which is well-studied and which does not spread in the air.