How Zika Virus Spreads

Zika is caused by a virus found in many parts of the world. It reaches people in three main ways — mosquito bites, sex, and from a pregnant woman to her fetus — and, less often, through infected blood or tissue or in a laboratory. A person can pass the virus on before symptoms begin, while they are ill and after symptoms clear.

The virus

Zika virus belongs to genus Orthoflavivirus in the family Flaviviridae and carries its genome as single-stranded RNA. Its particles measure 40 nm across, with an outer envelope around a dense inner core. Its relatives include the viruses behind dengue, yellow fever, Japanese encephalitis and West Nile.

Zika virus under the electron microscope.

The three main routes: mosquitoes, sex, and mother to baby.

Mosquito bites

Most infections come from the bite of an infected Aedes aegypti or Aedes albopictus mosquito. A mosquito picks up the virus by biting an infected person while it is in their blood, which is usually only during the first week of infection, and can then infect others it bites.

No vaccine or medicine prevents Zika. Avoiding mosquito bites is the best protection.

Sex

A person with Zika can give it to sex partners through vaginal, anal or oral sex or shared sex toys — including someone who has no symptoms at the time, or who carries the virus and never develops any. Condoms lower the risk. The virus lasts longer in semen than in other body fluids, so the periods during which men and women can pass it on differ. Clinicians can find more on sexual transmission.

Pregnancy and birth

An infected pregnant woman can pass Zika to her fetus during pregnancy (in utero transmission) or around delivery (perinatal transmission), whether or not she has symptoms. Perinatal transmission can happen when infection occurs within about 2 weeks of delivery. Infection during pregnancy can cause serious birth defects and is linked to other pregnancy problems. In U.S. states and territories, about 5% of babies born after a confirmed or possible Zika infection in pregnancy had Zika-associated birth defects.

Breastfeeding. Zika has been found in breast milk, and possible infections have been seen in breastfed babies, but spread through milk has not been confirmed. Current evidence suggests that breastfeeding's benefits outweigh that risk, so CDC encourages women to breastfeed even if they were infected or lived in or traveled to an area with Zika risk.

Blood, tissue and laboratories

• No blood-transfusion case has been confirmed in the United States; Brazil has reported spread through platelet transfusions.
• Some human cell and tissue products — umbilical cord blood, gestational tissue and reproductive tissue — can hold the virus months after infection, so donors may need to wait before giving blood or tissue.
• A few laboratory-acquired infections have been reported, not all with a clear route.
• No spread in U.S. healthcare settings has been identified.

Two transmission cycles

Mosquitoes and primate hosts — human or not, depending on the cycle — keep Zika in circulation.

In the enzootic, or sylvatic, cycle, forest-dwelling Aedes mosquitoes are infected by feeding on non-human primates and pass the virus to other primates, human and non-human.

When the virus reaches people with no immunity to it, an urban cycle can start: city-dwelling Ae. aegypti and Ae. albopictus bite infected people and carry the virus to others. These mosquitoes usually breed in household containers that hold water, bite by day and by night, and feed both indoors and outdoors near homes. In the Pacific Islands, Ae. hensilli may also play a part.

Spread from person to person in utero, around birth and through sex has been documented, as has probable spread through transfusion; spread through breast milk remains possible.

Sources

• Centers for Disease Control and Prevention: "How Zika Spreads."