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  • Zika virus is transmitted to people primarily through the bite of an infected Aedes species mosquito (A. aegypti and A. albopictus). These are the same mosquitoes that spread dengue, chikungunya, and yellow fever viruses.
  • The mosquitoes typically lay eggs in and near standing water in things like buckets, bowls, animal dishes, flower pots and vases. 
  • They prefer to bite people, and live indoors and outdoors near people.
  • Mosquitoes that spread Zika are aggressive daytime biters.They can also bite at night.
  • Mosquitoes become infected when they feed on a person already infected with the virus. Infected mosquitoes can then spread the virus to other people through bites.

Sexual Transmission

  • What we know
    • Zika can be passed through sex from a person who has Zika to his or her sex partners. Sex includes vaginal, anal, oral sex, and the sharing of sex toys.
    • Sexual exposure includes sex without a condom with a person who traveled to or lives in an area with Zika.
    • Zika can be passed through sex, even if the person does not have symptoms at the time. It can be passed from a person with Zika before their symptoms start, while they have symptoms, and after their symptoms end. It may also be passed by a person who has been infected with the virus but never develops symptoms.
    • Current research shows that Zika can remain in semen longer than in other body fluids, including vaginal fluids, urine, and blood.
  • What we don't know
    • We don’t know if Zika can be passed through saliva during kissing.
    • We don’t know if Zika passed to a pregnant woman during sex has a different risk for birth defects than Zika transmitted by a mosquito bite.

Vertical Transmission

  • A pregnant woman can pass Zika virus to her fetus during pregnancy. Zika is a cause of microcephaly and other severe fetal brain defects. Infection during pregnancy has also been linked to adverse outcomes including pregnancy loss and eye defects, hearing loss, and impaired growth in infants. The full range of other potential health problems that Zika virus infection during pregnancy may cause is still under investigation.
    • Birth defects could include microcephaly, calcium deposits in the brain indicating possible brain damage, excess fluid in the brain cavities and surrounding the brain, absent or poorly formed brain structures, abnormal eye development, or other problems resulting from damage to the brain that affects nerves, muscles and bones, such as clubfoot or inflexible joints.
    • Pregnancy losses could include miscarriage, stillbirths, or terminations with evidence of the birth defects mentioned above.
  • To date, there are no reports of infants getting Zika virus through breastfeeding. Because of the benefits of breastfeeding, mothers are encouraged to breastfeed even in areas where Zika virus is found.
  • No evidence exists to suggest a non-pregnant woman who is infected with the Zika virus, whether symptomatic or not, will run risks of Zika related birth defects with future pregnancies.
  • To understand more about Zika virus infection, CDC established the US Zika Pregnancy Registry and is collaborating with state, tribal, local, and territorial health departments to collect information about pregnancy and infant outcomes following Zika virus infection during pregnancy.

Accidental Exposure

  • CDC is assisting in the investigation of a case of Zika in a Utah resident who is a family contact of an elderly Utah resident who died in late June of unknown causes but was subsequently confirmed to have had a Zika virus infection. The deceased patient had traveled to an area with Zika and lab tests showed he had uniquely high amounts of virus—more than 100,000 times higher than seen in other samples of infected people—in his blood. Laboratories in Utah and at the Centers for Disease Control and Prevention (CDC) reported evidence of Zika infection in both Utah residents. The living family contact has fully recovered from the Zika infection.
  • A University of Pittsburgh lab worker accidentally stuck herself with a needle on May 23, 2016 while working with the Zika virus, resulting in what appears to be the first known Zika infection through accidental needle stick. The employee subsequently exhibited symptoms associated with Zika. Whether she was pregnant at time of exposure was not indicated.


  • On October 20, 2016 news sources reported that the FDA has confirmed several units of donated blood in Florida have tested positive for Zika. As of this date, no transmission of the virus in the United States has been confirmed through transfusions.
  • On August 26, 2016 the Food and Drug Administration (FDA) issued a revised guidance recommending universal testing of donated Whole Blood and blood components for Zika virus in the U.S. and its territories.
    • Because all blood donations are now screened, donor screening as previously outlined in FDA’s February 2016 guidance can be discontinued.
    • If a donor volunteers a recent history of Zika infection, blood or blood components from that individual must not be collected. Such a donor should wait 120 days after a positive viral test or the resolution of symptoms, whichever timeframe is longer before donating.
  • The American Red Cross is following FDA blood donation guidance.

Zika and Animals

  • Nonhuman primates (apes and monkeys) have shown the ability to become infected with Zika virus, either naturally or experimentally.
  • At this time, there is no evidence that Zika virus is spread to people from contact with animals.
  • There have not been any reports of pets or other types of animals becoming sick with Zika virus.

Zika and Guillain-Barré Syndrome (GBS)

  • An increase in GBS has been observed in areas where a Zika virus epidemic has been documented (e.g., in French Polynesia and Brazil).
  • A direct causal relationship, however, has not been established between Zika viral infection and GBS.
  • Prior infection with dengue or genetic factors could contribute to or increase cases of GBS.
Map - How the Zika virus spread

Areas of Current Zika Transmission (CDC, 12/14/16):

Areas of Current Zika Transmission (CDC, 5/12/16)

Zika Cases in the United States (CDC, 05/03/2017):

Zika Cases in the US

US States (CDC, 05/03/2017):

  • Locally acquired vector-borne cases reported: 224
  • Travel-associated Zika virus disease cases reported: 4973
  • Other:
    • Laboratory acquired cases reported: 1
    • Sexually transmitted: 46
    • Congenital infection: 29
    • Unknown: 1
  • Total: 5274

US Territories (CDC, 05/03/2017):

  • Locally acquired cases reported: 36431
  • Travel-associated cases reported: 143
  • Total: 36574 (In local Zika endemic areas, cannot determine mosquito vs sexual transmission)

Pregnant Women with Any Laboratory Evidence of Possible Zika Virus Infection (CDC, 04/25/2017):

  • US states and DC: 1793
  • US territories: 3700

Estimated range of Aedes aegypti and Aedes albopictus mosquitoes in the United States, CDC

Estimated range of Aedes aegypti and Aedes albopictus mosquitoes in the United States, CDC

Other Resources: