Causes and Risk Factors of the Zika Virus

The Zika virus is transmitted by mosquito bites. It can also be passed from a pregnant mother to an unborn baby, transmitted from person to person through unprotected sex, and infected blood may pose a small risk as well.

By understanding how the Zika virus is passed, you will be better able to protect yourself and others from harm.

Zika virus risk factors
© Verywell, 2018 

Mosquito Transmission Risk

The Zika virus is a member of the virus family Flaviviridae and is closely related to other mosquito-borne viruses such as those that cause dengue fever, yellow fever, and Japanese encephalitis.

The primary carrier of the virus, the Aedes aegypti mosquito, is unusual in that it is most active during the daytime hours. It thrives in subtropical and tropical climates and can be found in much of South America, Central America, Central and East Africa, India, Southeast Asia, and northern Australia. In the United States, the mosquito is found mostly along the Gulf Coast running from Florida to Texas.

Mosquito bites occur most commonly during the spring and summer months when the insects are actively breeding. It only takes one bite for infection to occur. The virus can quickly move from the skin into the bloodstream and spread throughout the body.

While most cases of Zika virus infection are mild or asymptomatic (without symptoms), the virus can rarely lead to a serious complication known as Guillain-Barré syndrome in which the body's immune system attacks its own nerve cells. The disorder is most likely when a Zika infection lasts for more than a week and is accompanied by persistent fever.

Pregnancy Risk

While a Zika infection is usually mild and uneventful, it can have serious consequences if it is passed to a developing fetus during the early stages of pregnancy.

The virus increases the risk of miscarriage and stillbirth, and it increases the risk of severe malformations in the newborn baby. The most serious concern is microcephaly, a rare and irreversible birth defect in which a baby is born with an abnormally small head and brain.

While scientists do not yet fully understand how the infection causes these effects, it appears that the virus is able to breach the placenta during the early part of the first trimester of pregnancy when fetal stem cells are just starting to develop into the brain, heart, and other vital organs.

The risk of microcephaly appears to be highest during the first trimester. By the second and third trimesters, the risk of microcephaly will have decreased to near-negligible levels, according to research from the Center for Disease Control and Prevention. Still, babies born to mothers with Zika can have serious neurological issues regardless of the trimester when she was infected.

Overall, the risk of microcephaly in affected pregnancies is between 1% and 13%.

Sexual Transmission Risk

While the Zika virus is considered a mosquito-borne illness, early surveillance of the disease revealed that some infections had occurred in climates where mosquito infestations were unlikely. Further investigation revealed that many of these infections were being passed between sexual partners, and mostly from males to females.

According to evidence published in the New England Journal of Medicine, the Zika virus is able to persist in semen even longer than it can in mosquitoes, increasing the potential for male-to-female transmission. By contrast, the virus cannot thrive in either saliva or vaginal secretions, making it less likely to be transmitted from females to males.

Based on the current evidence, the Zika virus can be passed from a recently infected partner through oral, vaginal, or anal sex whether there are symptoms or not. The sharing of sex toys may also pose a risk.

Blood Transfusion Risk

The risk that the Zika virus poses to the blood supply is unclear. While there have been several cases in Brazil linked to platelet transfusions (typically used to treat people who have hemophilia or who are undergoing cancer chemotherapy), there have been no similar occurrences elsewhere.

In July 2018, the U.S. Food and Drug Administration announced revised guidelines on the testing of donated blood and blood components for Zika virus. Given the significant decrease in cases of Zika virus infection in the U.S. and its territories, instead of testing individual donations, the FDA recommends testing pooled donations.

Regional Risk

As of July 2019, the World Health Organization (WHO) reported that 87 countries and territories have had evidence of mosquito-borne transmission of Zika virus (ZIKV) distributed across four of the six WHO Regions (African Region, Region of the Americas, South-East Asia Region, and Western Pacific Region). The incidence of ZIKV infection in the Americas peaked in 2016 and declined substantially throughout 2017 and 2018. Zika virus transmission has been found in all countries in the Region of the Americas except mainland Chile, Uruguay, and Canada.

Meanwhile, as of July 2017, non-mosquito-borne infections, presumed to be sexually transmitted, were reported in 13 countries: Argentina, Canada, Chile, France, Germany, Italy, Netherlands, New Zealand, Northern Ireland, Peru, Portugal, Spain, and the United States.

The WHO further reported that infection with ZIKV continues to carry the risk of Guillain-Barré Syndrome and adverse pregnancy outcomes including increased risk of preterm birth, fetal death or stillbirth, and congenital Zika syndrome (CZS), which can include microcephaly, abnormal brain development, limb contractures, eye abnormalities, brain calcifications, and other neurologic manifestations.

Frequently Asked Questions

  • What causes Zika virus disease?

    Zika virus disease is caused by a virus that belongs to the Flaviviridae family, which also includes dengue fever, yellow fever, Japanese encephalitis, and West Nile virus. All are largely considered mosquito-borne diseases.

  • How is Zika virus spread?

    Zika virus is mainly spread by mosquitoes, namely the Aedes mosquito. The virus can also be transmitted during sex, mostly from males to females as the virus can persist for months in semen. Zika can also be passed from mother to child during pregnancy and, on rare occasions, through a blood transfusion.

  • How is Zika virus not transmitted?

    You cannot get Zika from hugging, kissing, toilet seats, sharing utensils, or touching objects. The virus cannot be passed in saliva or via respiratory droplets, so coughing or sneezing poses no risk.

  • Where are you most likely to get Zika virus?

    Zika outbreaks mainly occur in subtropical or tropical climates where the Aedes mosquito thrives, such as the Caribbean, South and Central America, the Pacific region, Africa, and parts of south and southeast Asia. In the United States, most cases were seen around the Gulf Coast from Florida to Texas during the 2016-2017 outbreak.

  • When are you most likely to get Zika virus?

    The potential for a Zika outbreak is highest during the summer months (July through September), particularly when accompanied by wet weather. The Aedes mosquito is most active during the day (typically two hours after sunrise and several hours before sunset).

  • Can Zika virus cause birth defects?

    Yes. During the 2016-2017 outbreak, between 5 and 10% of babies born to mothers with confirmed Zika infections had birth defects. Among the most serious concerns is microencephaly, characterized by an abnormally small brain. Other possible defects include clubfoot, brain malformations, and retinal or macular eye damage.

  • What is the risk of Zika-associated birth defects?

    Studies suggest that between 1 and 13% of mothers infected with Zika in Brazil—considered ground zero for the 2016-2017 pandemic—had children with microencephaly.

17 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Centers for Disease Control and Prevention. Zika transmission.

  2. Centers for Disease Control and Prevention. Congenital Zika Syndrome & Other Birth Defects.

  3. Johansson MA, Mier-y-Teran-Romero L, Reefhuis J, Gilboa SM, Hills SL. Zika and the risk of microcephalyN Engl J Med. 2016;375(1):1-4. doi:10.1056/NEJMp1605367

  4. McCarthy M. Microcephaly risk with Zika infection is 1-13% in first trimester, study shows. BMJ. 2016;353:i3048. doi:10.1136/bmj.i3048

  5. Paz-Bailey G, Rosenberg ES, Doyle K, Munoz-Jordan J, Santiago GA, Klein L, Perez-Padilla J, Medina FA, Waterman SH, Gubern CG, Alvarado LI, Sharp TM. Persistence of Zika virus in body fluids - Final report. N Engl J Med. 2017 Feb 14;379(13):1234-1243. doi:10.1056/NEJMoa1613108

  6. Food and Drug Administration. FDA announces revised guidance on the testing of donated blood and blood components for Zika virus.

  7. World Health Organization. Zika Epidemiology Update.

  8. World Health Organization. Situation Report: Zika Virus, Microcephaly, Guillan-Barré Syndrome.

  9. Noorbakhsh F, Abdolmohammadi K, Fatahi Y, et al. Zika virus infection, basic and clinical aspects: a review article. Iran J Public Health. 2019;48(1):20-31.

  10. Atkinson B, Thorburn F, Petridou C, et al. Presence and persistence of Zika virus RNA in semen, United Kingdom, 2016. Emerg Infect Dis. 2017;23(4):611-5. doi:10.3201/eid2304.161692

  11. National Health Service (UK). Zika virus.

  12. Newman CM, Dudley DM, Aliota MT, et al. Oropharyngeal mucosal transmission of Zika virus in rhesus macaques. Nat Commun. 2017;8(1):169. doi:10.1038/s41467-017-00246-8

  13. Centers for Disease Control and Prevention. Zika in the US.

  14. Monaghan AJ, Morin CW, Steinhoff DF, et al. On the seasonal occurrence and abundance of the Zika virus vector mosquito Aedes aegypti in the contiguous United States. PLoS Curr. doi:10.1371/currents.outbreaks.50dfc7f46798675fc63e7d7da563da76

  15. Centers for Disease Control and Prevention. Dengue and the Aedes aegypti mosquito.

  16. Centers for Disease Control and Prevention. Data statistics on Zika and pregnancy.

  17. Reynolds MR, Jones AM, Petersen EE, et al. Vital signs: update on Zika virus-associated birth defects and Evaluation of all U.S. infants with congenital Zika virus exposure - U.S. Zika pregnancy registry, 2016MMWR Morb Mortal Wkly Rep. 2017;66(13):366-73. doi:10.15585/mmwr.mm6613e1

By Elizabeth Boskey, PhD
Elizabeth Boskey, PhD, MPH, CHES, is a social worker, adjunct lecturer, and expert writer in the field of sexually transmitted diseases.