Four viruses that ride the bite.

Zika

Most infected people have no symptoms, or a mild illness with fever, rash, red eyes, and joint and muscle pain that lasts a few days. The danger is in pregnancy. Infection can pass to the fetus and cause microcephaly and other serious birth defects, known as congenital Zika syndrome. Zika is also linked to Guillain-Barré syndrome and can spread through sex.

Where it stands

The 2015–16 epidemic that began in Brazil spread across the Americas and led WHO to declare a global health emergency. Large outbreaks have since died down, and cases in the continental US are rare and mostly linked to travel. It still circulates at low levels in parts of the Americas, Africa and Asia. Spreading or controlled?

What to do

• If you're pregnant, check current travel notices before visiting areas with Zika.
• Prevent bites day and night, and follow health-authority guidance on safe sex after travel.
• See a doctor if you develop symptoms during or after travel, especially if you're pregnant.

Dengue

About one in four infected people gets sick. Symptoms include high fever with nausea or vomiting, rash, pain behind the eyes, and severe muscle, joint and bone aches. Most recover in about a week, but roughly 1 in 20 sick people develop severe dengue, which can cause shock, internal bleeding and death. There are four types of the virus. A second infection with a different type raises the risk of severe disease.

Where it stands

Dengue is the most active threat this mosquito carries. 2024 set a record, with 14.4 million reported cases and over 11,000 deaths in more than 100 countries. The Americas accounted for over 90% of cases, and Brazil alone reported more than 10 million. Local transmission happens regularly in Puerto Rico and occasionally in Florida, Texas and California.

Vaccines

WHO recommends the two-dose TAK-003 (Qdenga) vaccine for children aged 6–16 in high-transmission areas. Brazil approved a single-dose vaccine (Butantan-DV) in 2025. In the US, the older Dengvaxia vaccine was only for children with a confirmed past infection and is being discontinued. Ask a doctor what's available where you live.

Yellow Fever

Most people have no symptoms or a short illness with fever, chills, headache, back pain and vomiting. A minority recover briefly and then develop severe disease, with jaundice (the "yellow"), bleeding and organ failure. Between 30% and 60% of those people die. There is no specific treatment.

Where it stands

It once devastated Atlantic port cities from Philadelphia to Lisbon. Today it is controlled by an excellent vaccine, but it remains a threat in tropical Africa and South America. In 2025 South America saw its biggest surge in years, mostly spread by forest mosquitoes. Ae. aegypti is the species that turns it into an urban epidemic, which is why vaccination coverage in cities matters. Read the history →

What to do

• Get vaccinated at least 10 days before travel to a risk area. Some countries require proof at the border.
• For most people, one dose gives lifelong protection.

Chikungunya

It starts suddenly with high fever and severe joint pain, often in the hands and feet, plus headache, muscle pain and rash. Most people feel better within a week, but joint pain can last for months or years. Deaths are rare but more likely in newborns, adults over 65, and people with heart disease, diabetes or high blood pressure.

Where it stands

Chikungunya causes explosive outbreaks that can infect a large share of a population quickly. 2025 brought large outbreaks on Indian Ocean islands and in southern China, and locally spread cases in Europe. In the US most cases are travel-related.

Vaccines

A vaccine (Vimkunya) was approved in the US in 2025 for people aged 12 and older travelling to outbreak areas. The US licence of an earlier vaccine was suspended in 2025 over safety concerns in older adults. Talk to a travel clinic.