Zika: What you need to know

The spread of the Zika virus across the Americans has been declared an international health emergency, as was the Ebola outbreak in West Africa. Here’s what you need to know about it.

Q: What is Zika?

A: Zika is a virus transmitted primarily through the bite of an infected Aedes mosquito, a species that also spreads dengue and yellow fever. It takes its name from the Zika forest in Uganda, where it was first isolated from a monkey in 1947.

The disease is common in parts of equatorial Africa and Southeast Asia. Outbreaks have also been reported in the Pacific islands. But the disease did not begin to spread widely in the Americas until May, when an outbreak was reported in Brazil. It has now spread to 23 countries and territories in the region.

Q: Why is it now a crisis?

A: According to the U.S. Centers for Disease Control and Prevention, only about one in five people infected with the Zika virus fall ill, and the symptoms are usually mild: fever, rash, joint pain and red eyes.

The arrival of the virus in Brazil, however, coincided with a sharp increase in a condition known as microcephaly — an unusually small head size and brain damage — in newborns.

Although there are a number of possible causes, including exposure to certain chemicals, bacteria and radiation, the correlation with Zika has been sufficiently high to raise alarm. El Salvador, for example, has advised its population to put off having children for two years.

Q: How did it get here?

A: That's unclear. One theory is that the virus entered Brazil around the time of the 2014 World Cup soccer tournament, when hundreds of thousands of people were visiting from around the globe. Some experts have also suggested that El Nino weather patterns, which are expected to significantly increase mosquito populations in many areas, could be playing a role.

Q: Why has the virus spread so quickly?

A: There has been little previous exposure to Zika in the Americas, so people lack immunity. It is also spread by mosquitoes of the Aedes species, which can breed in a pool of water as small as a bottle cap and usually bite during the day. The aggressive yellow fever mosquito, Aedes aegypti, has spread most Zika cases, but that mosquito is common in the United States only in Florida, along the Gulf Coast, and in Hawaii — although it has been found as far north as Washington, D.C., in hot weather.

The Asian tiger mosquito, Aedes albopictus, is also known to transmit the virus, but it is not clear how efficiently. That mosquito ranges as far north as New York and Chicago in summer.

Q: Is the United States at risk?

A: The WHO says transmission will likely spread to all the countries and territories in the region where Aedes mosquitoes are found, including the U.S. mainland.

The CDC says it has been aware of the threat for some time and has been taking steps to limit any possible spread. That includes alerting health care providers and the public about the virus, and providing state laboratories with diagnostic tests.

Q: Besides mosquito bites, are there other ways to catch Zika?

A: Although the role of Aedes mosquitoes is well documented, the evidence about other transmission routes is limited.

Scientists say the virus can be transmitted from a mother to her baby during pregnancy or around the time of birth, but there is currently no evidence to suggest that it can be spread through breast milk.

The virus can also be spread through sexual contact.

There has been a report of possible spread through blood transfusion. But this is thought to be an infrequent means of transmission.

Q: How can people protect themselves?

A: There is no vaccine or cure for the Zika virus, but there are steps that people can take to reduce the risk of infection:

• The World Health Organization and the CDC recommend reducing mosquito populations by eliminating breeding sites, especially in and around the places where people live. Containers that can hold even small amounts of water — buckets, flower pots, tires — should be emptied, cleaned or covered to prevent mosquitoes from breeding in them. Another option is to treat standing water with larvicide.

• Those living in or visiting Zika-affected areas are urged to protect themselves from mosquito bites by using insect repellent; wearing clothes (preferably light-colored) that cover as much of the body as possible; closing doors and windows, or using screens to keep out the insects; and sleeping under insecticide-treated bed nets, especially during the day when Aedes mosquitoes are most active.

• Pregnant women need to be especially careful to avoid mosquito bites. Women planning to travel to areas where Zika is circulating are being urged to consult a health care provider before traveling and upon return. If they believe they have been exposed to the virus, they should consult with their health care providers for close monitoring of their pregnancy, authorities say.