Global health officials are racing to better understand the Zika virus behind a major outbreak that began in Brazil last year and has since spread to many countries in the Americas.

The following are some questions and answers about the virus and current outbreak:

How do people become infected?

Zika is transmitted to people through the bite of infected female mosquitoes, primarily the Aedes aegypti mosquito, the same type that spreads dengue, chikungunya and yellow fever. The Pan American Health Organization (PAHO) said Aedes mosquitoes are found in all countries in the Americas except Canada and continental Chile, and the virus will likely reach all countries and territories of the region where Aedes mosquitoes are found.

How do you treat Zika?

There is no treatment or vaccine for Zika infection. Companies and scientists are racing to develop a safe and effective vaccine for Zika, but the World Health Organization (WHO) said it would take at least 18 months to start large-scale clinical trials of potential preventative shots.

How dangerous is it?

The PAHO said there is no evidence that Zika can cause death, but some cases have been reported with more serious complications in patients with pre-existing medical conditions.

It has also been associated with Guillain-Barre syndrome, a rare disorder in which the body’s immune system attacks part of the nervous system.

How is Zika related to microcephaly?

Much remains unknown about Zika, including whether the virus actually causes microcephaly in babies, a condition defined by unusually small heads that can result in developmental problems.

Research is under way in Brazil to confirm the suspected link to microcephaly, with initial findings expected within months, according to public health officials.

Brazil said it has confirmed 944 cases of microcephaly, and considers most of them to be related to Zika infections in the mothers. Brazil is investigating an additional 4,291 suspected cases of microcephaly.

Research in Brazil indicates the greatest microcephaly risk is associated with infection during the first trimester of pregnancy. Recent studies from other countries have shown evidence of Zika in amniotic fluid, placenta and fetal brain tissue.

What are the symptoms of Zika infection?

People infected with Zika may have a mild fever, skin rash, conjunctivitis, muscle and joint pain and fatigue that can last for two to seven days. But as many as 80 percent of people infected never develop symptoms. The symptoms are similar to those of dengue or chikungunya, which are transmitted by the same type of mosquito.

How can Zika be contained?

Efforts to control the spread of the virus focus on eliminating mosquito breeding sites and taking precautions against mosquito bites such as using insect repellent and mosquito nets. U.S. and international health officials have advised pregnant women to avoid travel to Latin American and Caribbean countries where they may be exposed to Zika.

How widespread is the outbreak?

Active Zika outbreaks have been reported in at least 39 countries or territories, most of them in the Americas, according to the U.S. Centers for Disease Control and Prevention (CDC). Brazil has been the country most affected. (http://1.usa.gov/1ovAJyh)

Africa (1): Cape Verde

Americas (33): Aruba, Barbados, Bolivia, Bonaire, Brazil, Colombia, Costa Rica, Cuba, Curaçao, Dominica, Dominican Republic, Ecuador, El Salvador, French Guiana, Guadeloupe, Guatemala, Guyana, Haiti, Honduras, Jamaica, Martinique, Mexico, Nicaragua, Panama, Paraguay, Puerto Rico, Saint Martin, Saint Vincent and the Grenadines, St. Maarten, Suriname, Trinidad and Tobago, U.S. Virgin Islands and Venezuela

Oceania/Pacific Islands (5): American Samoa, Marshall Islands, New Caledonia, Samoa, and Tonga

What is the history of the Zika virus?

The Zika virus is found in tropical locales with large mosquito populations. Outbreaks of Zika have been recorded in Africa, the Americas, Southern Asia and the Western Pacific. The virus was first identified in Uganda in 1947 in rhesus monkeys and was first identified in people in 1952 in Uganda and Tanzania, according to the WHO.

Can Zika be transmitted through sexual contact?

The World Health Organization (WHO) said sexual transmission is “relatively common” and has advised pregnant women not to travel to areas with ongoing outbreaks of Zika virus.

The US CDC is investigating about a dozen cases of possible sexual transmission. All cases involve possible transmission of the virus from men to their sex partners.

On Feb. 27, France said it had detected its first sexually transmitted case of Zika in a woman whose partner had traveled to Brazil.

British health officials reported Zika was found in a man’s semen two months after he was infected, suggesting the virus may linger in semen long after infection symptoms fade. The WHO has advised women, particularly pregnant women, to use condoms.

The PAHO said Zika can be transmitted through blood, but this is an infrequent transmission mechanism. There is no evidence Zika can be transmitted to babies through breast milk.

What other complications are associated with Zika?

The WHO says because no big Zika outbreaks were recorded before 2007, little is known about complications caused by infection. During an outbreak of Zika from 2013-14 in French Polynesia, national health authorities reported an unusual increase in Guillain-Barre syndrome. Health authorities in Brazil have also reported an increase in Guillain-Barre syndrome.

Long-term health consequences of Zika infection are unclear. Other uncertainties surround the incubation period of the virus and how Zika interacts with other viruses that are transmitted by mosquitoes, such as dengue.