Zika virus is a mosquito-borne virus in the same family as dengue, yellow fever, and West Nile virus. It spreads primarily through the bite of infected Aedes mosquitoes, the same species responsible for dengue. Most people who contract Zika never feel sick, but the virus poses serious risks during pregnancy because it can cause severe birth defects.
How Zika Spreads
The two main mosquito species that carry Zika are Aedes aegypti and Aedes albopictus, commonly known as the yellow fever mosquito and the Asian tiger mosquito. Both are aggressive daytime biters found in tropical and subtropical regions worldwide. Unlike many mosquitoes that feed at dusk, these species bite throughout the day, which makes avoidance harder.
Zika also spreads without mosquitoes. It can be transmitted through sex, even when the infected person has no symptoms. A person can pass the virus before symptoms appear, while they’re symptomatic, and after symptoms resolve. The virus has also been transmitted through blood transfusions (documented in Brazil through platelet transfusions) and from a pregnant person to their developing baby during pregnancy or around the time of delivery.
Symptoms and Timeline
After a bite from an infected mosquito, the incubation period is 3 to 14 days. When symptoms do appear, they’re typically mild: fever, rash, joint pain, red eyes, muscle pain, and headache. The illness usually lasts several days to a week. Most people recover without needing medical care, and hospitalization is rare.
The tricky part is that a large proportion of infected people never develop symptoms at all. This makes the virus easy to spread unknowingly, particularly through sexual contact or during pregnancy when neither partner realizes they’ve been infected.
Why Zika Is Dangerous During Pregnancy
Zika’s most serious consequence is what it does to a developing fetus. The virus directly targets the brain cells responsible for building the fetal brain, particularly during the first trimester. It attacks neural progenitor cells, the stem cells that eventually form the brain’s outer layer, and disrupts their ability to grow and divide normally. Infected cells stop progressing through their normal cycle, and many die off entirely. The result is a thinner cortex and a significantly smaller brain.
This is the mechanism behind microcephaly, the condition where a baby is born with an abnormally small head and underdeveloped brain. Beyond microcephaly, Zika infection during pregnancy has been linked to restricted fetal growth, fluid buildup in the brain, damage to the placenta, and in some cases pregnancy loss. These outcomes are collectively called congenital Zika syndrome.
The virus doesn’t just affect the fetus. In adults, Zika has been associated with Guillain-Barré syndrome, a condition where the immune system attacks the nerves, causing weakness and sometimes temporary paralysis. Cases of brain inflammation (meningoencephalitis) have also been reported, though both complications are uncommon.
How It’s Diagnosed
Testing for Zika depends on timing. Within the first seven days of symptoms, a molecular test (nucleic acid amplification) performed on a blood sample is the most reliable method. This test detects the virus’s genetic material directly. After seven days, antibody testing becomes the better option, since the body has had time to mount an immune response. Antibodies typically become detectable within the first week and can persist for months or even years.
For pregnant individuals, the testing window is wider. Specimens can be collected up to 12 weeks after symptoms start or after returning from an area with known Zika transmission. This extended window reflects the higher stakes of missing a diagnosis during pregnancy.
One complication with antibody testing: Zika antibodies can cross-react with dengue and other related viruses, sometimes producing unclear results. This is especially relevant in regions where multiple flaviviruses circulate simultaneously.
Treatment and Vaccine Status
There is no specific antiviral treatment for Zika. Managing the illness means treating symptoms: rest, fluids, and over-the-counter pain relief. The infection clears on its own in the vast majority of cases.
More than a decade after the major outbreaks, there is still no approved Zika vaccine. Multiple candidates are in development across several platforms, including inactivated virus, DNA-based, mRNA-based, and viral vector approaches. Some have progressed into clinical trials, but none has reached approval.
Prevention After Travel
Because Zika can linger in bodily fluids after the infection clears, the CDC recommends specific timelines for preventing sexual transmission after traveling to an area with Zika risk. Males should use condoms or abstain from sex for at least 3 months after returning, even if they never developed symptoms. Females should do the same for at least 2 months. These timelines start from the date of return, the date symptoms began, or the date of diagnosis, whichever applies.
Standard mosquito prevention also applies in affected areas: using EPA-registered insect repellent, wearing long sleeves and pants, staying in places with screens or air conditioning, and eliminating standing water where Aedes mosquitoes breed.
Where Zika Is Found Today
The large outbreaks that swept through the Americas in 2015 and 2016 are over. As of now, there are no active CDC Zika travel health notices for any country. That said, the virus hasn’t disappeared. It continues to circulate at lower levels in many tropical and subtropical countries across the Americas, Africa, Southeast Asia, and the Pacific Islands. These regions are classified as having current or past transmission, meaning the risk persists even though large-scale outbreaks aren’t occurring.
Detection and reporting of new cases in these areas can be delayed, so the absence of an active travel notice doesn’t mean zero risk. For anyone planning travel to tropical regions, particularly those who are pregnant or trying to conceive, checking current CDC risk classifications before departure remains important.