A once-rare mosquito-borne virus called Zika is spreading rapidly through the Americas since arriving in Brazil last May, with a case confirmed in Texas this week in a traveler returning from Latin America. Health officials are alarmed because of a possible link between the virus and a birth defect called microcephaly, which causes babies to be born with abnormally small heads and potential developmental problems.

Here’s what you need to know.

1. What’s the Zika virus?

It’s a virus that can make people sick with rash, fever, joint pain, and pinkeye, according to the Centers for Disease Control and Prevention. It’s related to other mosquito-borne illnesses including dengue and chikungunya that are considered rising threats as global travel increases their reach. About one in five people infected develop symptoms, the CDC says, which begin a few days after being bitten by Zika-carrying mosquitoes. The illness generally isn’t severe, and it lasts as long as a week. But Brazilian authorities have identified at least seven deaths potentially linked to the virus in newborns and adults. Some press reports have put the number of infant deaths at 38.

2. Why is it a concern?

Health officials are worried that the Zika outbreak in Brazil may be linked to a dramatic increase in babies being born with unusually small heads. Brazil usually records only about five to six cases of microcephaly for every 100,000 live births. As of the end of November, the rate was 20 times that: nearly 100 cases for every 100,000 births, according to the World Health Organization.

Health authorities haven’t definitely confirmed that Zika is behind the spike in kids being born with abnormally small heads. But they don’t have another explanation for the spike in microcephaly. “The question is, is this a new phenomena?” said Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota.

3. How does it spread?

Zika can be transmitted when someone with the virus is bitten by a mosquito, and then that mosquito bites another human. In particular, it’s spread by the Aedes mosquito, which is also the vector for yellow fever, dengue, and chikungunya. The virus can move to new areas when mosquitoes hitch a ride on travelers or cargo, or when infected patients travel to new territory and are bitten by mosquitoes there. “We’re moving mosquitoes around the world at will right now,” Osterholm said.

In much of the world, trash dumps and water systems can create conditions that breed mosquitos: A discarded fast-food wrapper in a ditch that forms a puddle can be a breeding ground for bugs, Osterholm said. Growing urban populations allow them to spread viruses among humans rapidly.

The virus can also spread from an infected mother to a newborn, but the CDC calls this rare. It may be possible as well to spread through blood transfusions or sexual contact.

4. Where did it come from?

The Zika virus was first identified in Uganda in 1947. For a long time it was an obscure disease with only occasional infections in humans, according to an essay in the New England Journal of Medicine this week. “Its current explosive pandemic reemergence is therefore truly remarkable,” top officials from the National Institute of Allergy and Infectious Diseases wrote. It moved through Africa and Asia to Pacific islands. In French Polynesia, it was associated with a spike in cases of a rare neurological problem known as Guillain-Barré syndrome. The virus reached Brazil last year; it’s now spread throughout Latin America. The first locally transmitted case in Puerto Rico was identified in December. The WHO lists 13 countries in the Americas, including Puerto Rico, where transmission has been reported.

5. What can I do to protect myself?

There’s no known vaccine or treatment for Zika, though people who’ve been infected may have immunity. Part of the reason it spread so quickly in Latin America is that the population had never seen it before, Osterholm said. The best prevention strategy is taking steps to avoid getting bitten by mosquitoes. The CDC advised pregnant women in any trimester to consider delaying travel to any of the 14 countries in Latin America or the Carribean where the Zika virus is spreading. Women who must travel should consult with their doctors, and all travelers should follow precautions to avoid mosquito bites, including using repellant, screens, and long sleeves and pants.

Osterholm said Zika transmission is likely to come to the mainland U.S. “We shouldn’t be surprised,” he said. “We knew it was just a matter of time.” The good news: The U.S. has far better mosquito control than many of the impacted countries, so extended transmission here is unlikely. “Will it take off in the United States? No,” Osterholm said.