1. What is Yellow Fever?

Yellow Fever is a viral disease caused by the yellow fever virus (Flavivirus), transmitted mainly by the bites of infected mosquitoes (e.g., Aedes, Haemagogus, Sabethes). Infection most often occurs in tropical and subtropical climates when mosquitoes are active (important: many species transmitting yellow fever bite during the day).

Source of pathogen (reservoir):

in the "jungle" (sylvatic) cycle – primarily monkeys,
in the "urban" cycle – humans (and the mosquito transmits the virus from human to human).

The incubation period is usually 3–6 days.

Most at risk are: unvaccinated people traveling to risk regions (especially for longer stays, trips "into the field", into the jungle or rural areas), as well as people who cannot effectively avoid mosquito bites.

2. Symptoms of Yellow Fever

In many people, the disease is mild or even asymptomatic. If symptoms appear, they are most often:

fever, chills, weakness
headache and muscle pain
nausea, vomiting, body aches

Symptoms in a typical course may resolve after 3–4 days. However, in some infected people (about 15%), a severe phase of the disease occurs: high fever returns, jaundice, bleeding (e.g., from nose/mouth, from the digestive tract) appear, and organ failure and shock may develop. In the severe phase, the risk of death is very high (WHO states that approx. 50% of patients in this phase die within 7–10 days).

Urgent medical consultation (or ER) is necessary when after a stay in a risk region the following occur:

jaundice (yellowing of the skin and whites of the eyes),
bleeding, bloody vomit or tarry stools,
severe abdominal pain, significant weakness, disturbances of consciousness,
high fever persisting or recurring.

3. Where does Yellow Fever occur?

Yellow Fever occurs in the tropical and subtropical zone, mainly in:

Sub-Saharan Africa,
Central and South America.

WHO indicates that 27 African countries and 13 Latin American countries are classified as areas of high risk of epidemic outbreaks. For travelers, trips to Amazon regions and forest/peri-forest areas as well as connecting flights through countries with transmission risk are particularly important.

Seasonality: in Africa, transmission can be seasonal – in West Africa, increased risk is associated with the middle of the rainy season (around August) and the peak at the beginning of the dry season (around October). However, the risk may also occur outside the "peak season", depending on the region and epidemiological situation.

4. How to protect yourself against Yellow Fever?

A. General prevention

Since yellow fever is transmitted by mosquitoes (often active during the day), limiting bites is key:

use repellents (according to instructions, renew application regularly),
wear long sleeves and trousers, preferably light-colored clothes,
choose accommodation with air conditioning or screens in windows/doors,
limit mosquito breeding places (e.g., standing water near accommodation),
remember that bed nets may be less effective if mosquitoes bite mainly during the day – but can still help in some conditions.

B. Vaccination

This is the most effective form of protection.

Type of vaccine: yellow fever vaccine is live, attenuated (weakened virus).
For whom recommended: primarily for people traveling to risk areas and those who are required to confirm vaccination upon entry to a given country.
Schedule: standard 1 dose (in people from 9 months of age; in younger children, vaccination is generally not recommended / has contraindications).
Booster doses: WHO indicates that they are not needed – one dose provides long-term protection.
When does it work and how long does it protect: immunity develops in 80–100% of vaccinated people within 10 days, and in >99% within 30 days; protection is lifelong.
Yellow Card (ICVP): the international certificate of vaccination becomes valid 10 days after vaccination and is recognized as valid for the life of the vaccinated person.
Important notes and contraindications (examples): vaccination is usually not recommended, among others, in infants <9 months, people with severe egg allergy, severe immune deficiency, or thymus diseases; in people >60 years of age, the decision requires particularly careful assessment of benefits and risks.

5. Summary

Yellow Fever is a dangerous viral disease transmitted by mosquitoes in Sub-Saharan Africa and Central and South America. In most people, it runs mildly, but in some, it can pass into a severe phase with jaundice, bleeding, and organ failure, which can be fatal. Vaccination is key especially for people traveling to risk regions and where an entry certificate is required. It is best to perform it at least 10 days before departure (practically: it is worth aiming for 2–4 weeks earlier, to calmly manage immunity and formalities).

1. What is Yellow Fever?