Yellow fever is a serious viral infection spread by mosquitoes, capable of causing severe illness and even death, yet also entirely preventable through vaccination and protection from mosquito bites.

Yellow fever is a disease that has shaped history and continues to pose significant health risks in tropical regions of the world. The infection gets its name from one of its most striking symptoms, where the skin and eyes turn yellow due to liver damage. While many people infected with the virus experience only mild symptoms or none at all, the disease can progress to a severe and life-threatening form that affects multiple organs in the body.

This mosquito-borne illness remains a pressing concern for global health security, particularly because it can spread rapidly when infected individuals travel to areas with dense mosquito populations. The good news is that yellow fever is vaccine-preventable, and a single dose of the vaccine provides lifelong protection for most people.

Where Yellow Fever Occurs in the World

Yellow fever is not found everywhere, but it remains a serious threat in specific parts of the world. The disease is endemic in tropical and subtropical areas, meaning it regularly occurs in these regions. Twenty-seven countries in Africa and thirteen countries in Latin America are classified as high-risk areas for yellow fever outbreaks.^[1] These regions include parts of sub-Saharan Africa, which is the area below the Sahara desert, and territories in Central and South America, including countries such as Argentina, Bolivia, Brazil, Colombia, Ecuador, French Guiana, Guyana, Panama, Paraguay, Peru, Suriname, Trinidad and Tobago, and Venezuela.^[3]

Approximately ninety percent of the estimated 200,000 cases of yellow fever each year occur in Africa.^[4] Since 1970, the disease has re-emerged as a public health threat in the Americas. In 2014, the virus began spreading beyond the Amazon basin, a change that some experts attribute to shifts in mosquito behavior and human activity patterns.^[3] More recently, in 2025, health organizations issued alerts due to an increase in confirmed human cases in several countries in the region and changes in where the disease was appearing. While cases in 2024 were mainly concentrated in the Amazon region, by 2025 the disease had started spreading to areas outside this zone.^[3]

It is important to note that yellow fever is not found in the United Kingdom, Europe, Asia, Australia, New Zealand, or the Pacific Islands.^[4] The disease no longer occurs naturally in the United States, although in past centuries it caused devastating outbreaks that disrupted economies and decimated populations in North America and Europe.^[3]

How Many People Are Affected

Yellow fever represents a substantial burden on global health. An estimated 67,000 to 173,000 severe infections occur each year in Africa and the Americas, resulting in approximately 31,000 to 82,000 deaths annually, with most of this burden falling on Africa.^[1] In 2013, yellow fever was estimated to have caused 130,000 severe infections and 78,000 deaths in Africa alone.^[4]

Despite these large numbers, yellow fever is actually a very rare cause of illness in travelers from the United States and other non-endemic countries.^[2] However, fatal cases in unvaccinated travelers to Africa and South America continue to be reported, which underscores the importance of vaccination for those visiting risk areas.^[8]

What Causes Yellow Fever

Yellow fever is caused by a virus that belongs to a group called flaviviruses, which are viruses that share certain characteristics and are named after the Latin word for yellow. This particular virus has the scientific name Orthoflavivirus flavi and is closely related to the viruses that cause West Nile fever, St. Louis encephalitis, and Japanese encephalitis.^[4][5]

The yellow fever virus is what scientists call an RNA virus, meaning its genetic material is made of ribonucleic acid rather than DNA.^[4] This type of virus can infect humans, other primates such as monkeys, and several types of mosquitoes.^[4] The virus does not spread directly from person to person through contact, coughing, or other typical routes of human-to-human transmission.

How Yellow Fever Spreads

Yellow fever is transmitted to people primarily through the bite of an infected mosquito. The mosquitoes that carry and spread the virus are active biters during the daytime, though they can also bite at night.^[2][6] The main types of mosquitoes involved in spreading yellow fever include Aedes species, particularly Aedes aegypti, as well as Haemagogus and Sabethes species.^[1][5]

In cities and urban areas, the disease is spread primarily by Aedes aegypti, a type of mosquito found throughout the tropics and subtropics.^[4] These mosquitoes breed in various environments, including domestic settings around homes, forest areas (called sylvatic environments), and semi-domestic locations that are part rural and part residential.^[1] Tree-hole breeding mosquitoes transmit yellow fever especially during the rainy season when water pools provide ideal breeding grounds.^[5]

The yellow fever virus has three distinct transmission cycles that help explain how it moves between animals and humans. The jungle cycle involves transmission between non-human primates like monkeys and mosquitoes in forested areas. Humans become infected when they are bitten by infected mosquitoes while visiting or working in the jungle. The intermediate cycle occurs in African savannah regions and involves transmission at the border areas between jungle and human settlements. It may involve mosquitoes spreading the virus between monkeys and humans, or between humans themselves. The urban cycle begins when a person who contracted the virus in either the jungle or intermediate zones becomes infected and returns to an urban area. Since infected humans develop significant amounts of virus in their blood (a state called viremia), mosquitoes in cities can bite them and then transmit the virus to other people.^[5]

Who Is at Risk

Certain groups of people are at higher risk of contracting yellow fever based on where they live or travel. People who work or live in the jungles of subtropical and tropical Africa and South America are most affected by yellow fever.^[6] Travelers from the United States or other countries visiting these areas can be exposed to the disease through contaminated mosquitoes.

In certain parts of South America, infections happen less often overall, but they occur particularly among forestry and agricultural workers or travelers to jungle areas. Recent outbreaks, however, have been reported in and around major cities in Brazil, showing that the disease is not limited to remote jungle regions.^[21] In Africa, infections happen most frequently in the tropical areas of western and central Africa, but they also occur in cities and jungle areas.^[21]

Unvaccinated individuals are at the highest risk. Fatal cases of yellow fever continue to be reported in unvaccinated travelers, emphasizing that the vaccine is a critical protective measure.^[8] Anyone traveling to countries with a risk of yellow fever should discuss the need for vaccination with their healthcare provider.^[17]

What Symptoms to Expect

The symptoms of yellow fever can vary dramatically from person to person. Many people infected with the virus may have no symptoms at all, while others experience mild symptoms, and some develop severe, life-threatening illness.^[2][6]

After being bitten by an infected mosquito, there is an incubation period, which is the time between infection and when symptoms start to appear. For yellow fever, this period typically lasts three to six days, though symptoms can sometimes take longer to appear.^[1][3]

When symptoms do occur, they usually start suddenly. The initial symptoms of yellow fever include fever, headache, general body aches, nausea, vomiting, and weakness.^[1] Other early symptoms may include chills, severe headache, back pain that can be quite prominent, loss of appetite, fatigue (feeling very tired), muscle pain, restlessness, irritability, flushed face, and constipation.^[3][10] These symptoms are similar to flu and typically last for three to four days, then disappear within three to four days.^[1][3]

For most people, the infection ends after these initial symptoms as the virus is cleared from the body. Most people who develop symptoms improve within one week.^[10] For some who recover, weakness and fatigue might persist for several months.^[10]

However, approximately fifteen percent of people infected with yellow fever will develop a severe form of the disease.^[1] For about one out of seven people who have the initial symptoms, there will be a brief remission, which is a time when they feel better. This remission may last only a few hours or up to a day, followed by a return of symptoms that are much more severe.^[10]

The severe symptoms of yellow fever represent what is known as the toxic phase. This phase is characterized by the recurrence of high fever, jaundice (yellowing of the skin and eyes, which is where the disease gets its name), vomiting, bleeding from the mouth, nose, eyes, or stomach, organ failure, and shock.^[1] Patients may vomit blood that looks black and resembles coffee grounds.^[3][8] Other severe complications include pain in the upper part of the abdomen, bloody urine, dangerously low blood pressure, liver failure, kidney failure, seizures, and in the most severe cases, coma.^[3][6]

About fifty percent of patients who enter this toxic phase die within seven to ten days.^[1] Among those who develop severe disease overall, thirty to sixty percent die.^[6][10]

For those who survive yellow fever, the body typically develops lifelong immunity, meaning they are protected against future infections with the virus.^[6][10]

How to Prevent Yellow Fever

Prevention of yellow fever involves two main strategies: vaccination and protection from mosquito bites. Both approaches are essential for staying safe in areas where the disease is found.

Vaccination

A safe and effective vaccine against yellow fever exists, and it is the most important tool for preventing the disease.^[4] The yellow fever vaccine is a live, attenuated vaccine, meaning it contains a weakened form of the virus that cannot cause disease but can train the immune system to recognize and fight the real virus. This vaccine, known as YF 17D, has been in wide use for over sixty years.^[5][13]

A single dose of the yellow fever vaccine provides lifelong protection for most people and situations.^[1][7] The vaccine is safe and effective for life in the majority of cases. The Centers for Disease Control and Prevention (CDC) recommends the vaccine for people nine months old and older who are traveling to or living in areas at risk for yellow fever in Africa and South America.^[23]

It is important to have the vaccine at least ten days before traveling to an endemic area, as it takes time for the body to develop immunity.^[7][19] Some countries require travelers to show proof of vaccination against yellow fever with an International Certificate of Vaccination or Prophylaxis before they are allowed to enter.^[1][7]

The vaccine is safe for most people, but there are some exceptions. The vaccine should not be given to babies younger than six months old, pregnant women in most cases, or people with weakened immune systems.^[6] Healthcare providers can help determine if the vaccine is appropriate for each individual.

In the United States, the yellow fever vaccine can only be obtained from registered yellow fever vaccination centers, which are special clinics authorized by the World Health Organization to provide the vaccine and certification.^[7] Travelers must pay for the yellow fever vaccine, as it is not typically covered by standard health insurance for routine care.

Avoiding Mosquito Bites

Regardless of vaccination status, all travelers to areas where yellow fever is found should take steps to avoid mosquito bites.^[17] Protection from mosquitoes is crucial because it also helps prevent other mosquito-borne diseases.

There are several practical measures people can take to reduce their risk of mosquito bites. Using insect repellent is one of the most effective methods. Repellents that contain DEET are particularly effective, though DEET should be used cautiously in small children and label instructions should always be followed.^[23] Another option is to spray or soak clothing with an insect repellent containing permethrin, a chemical that kills or repels mosquitoes on contact.^[17]

Wearing protective clothing can create a physical barrier against mosquitoes. Long-sleeved shirts and long pants should be worn when outside, especially during early morning and early evening when mosquitoes are most active.^[7] Choosing light-colored clothing can also help, as mosquitoes are more attracted to dark colors.

Environmental measures are equally important. When possible, people should choose hotels or lodging with air conditioning or screens on windows and doors.^[17] If sleeping in an area without screens, using a mosquito net treated with insecticide is essential, and this protection should be used even during the day.^[7] Closing windows and doors whenever possible, or using blinds or screens, can help keep mosquitoes out of living spaces.^[7]

In areas with potential mosquito populations, avoiding going outside unnecessarily can also reduce exposure risk.^[23]

How the Disease Affects the Body

Understanding how yellow fever affects the body helps explain why the disease can be so dangerous. When an infected mosquito bites a person, the yellow fever virus enters the bloodstream and begins to multiply. The virus has the ability to infect and damage several vital organs, including the liver, kidneys, and heart.^[6][13]

The liver is often the organ most severely affected by yellow fever. When the virus damages liver cells, the liver cannot perform its normal functions properly. One of the liver’s important jobs is to process a substance called bilirubin, which comes from the breakdown of old red blood cells. When the liver is damaged, bilirubin builds up in the blood and tissues, causing the characteristic yellow color of the skin and eyes that gives the disease its name.^[3]

The kidneys can also be severely affected, leading to kidney failure. When the kidneys fail, they cannot filter waste products from the blood or regulate fluid balance in the body properly.^[6] This can lead to dangerous accumulations of toxins and imbalances in body chemistry.

One of the most serious complications of severe yellow fever is bleeding. The virus can cause problems with blood clotting, leading to bleeding from the gums, nose, mouth, eyes, stomach, and intestines.^[1] This bleeding happens because the virus damages blood vessels and interferes with the body’s ability to form clots.

Scientists believe that some of the most severe and potentially fatal complications of yellow fever may be due to what is called a “cytokine storm.” Cytokines are chemical messengers that immune cells use to communicate with each other. In severe infections, the immune system can sometimes overreact and release massive amounts of cytokines, which causes widespread inflammation and organ damage throughout the body.^[13] This overactive immune response, rather than the virus itself, may be responsible for much of the damage seen in fatal cases.

The terminal events in fatal yellow fever cases include shock, which is a life-threatening condition where blood pressure drops so low that organs do not receive enough blood and oxygen, and multi-organ failure, where several organ systems stop working at the same time.^[13]

Despite decades of research, much about the exact mechanisms of yellow fever pathology remains incompletely understood, particularly the events that lead to death in the most severe cases. This is one reason why developing specific treatments for yellow fever has been challenging.