Dengue fever is a mosquito-borne viral illness that affects millions of people worldwide each year, bringing fever, intense body pain, and sometimes life-threatening complications. Though often mild, this disease requires careful attention and timely medical care to ensure recovery.

Epidemiology

Dengue fever represents one of the most significant mosquito-borne viral diseases affecting human populations globally. The scale of this health challenge is substantial, with research estimating that nearly 400 million people become infected with the dengue virus each year, though remarkably, about 80% of these infections produce no symptoms at all.^[2] This means that many people carry and potentially spread the virus without ever knowing they are infected.

The disease has shown dramatic growth over recent decades. According to the World Health Organization, the occurrence of dengue has increased thirty-fold over the previous fifty years.^[16] This expansion has been particularly pronounced in the Americas, where in 2023 alone, there were 4.6 million reported cases resulting in 4,000 deaths. By late June 2024, over 9.7 million cases had been reported in the Americas—more than double the number throughout the previous year.^[16] These numbers continue to climb as temperatures rise globally, allowing the mosquitoes that carry the virus to expand into new geographic regions.

Currently, about half of the world’s population lives in areas where they are at risk of dengue infection.^[3] The disease has become endemic in over 100 countries across tropical and subtropical regions, including parts of Central and South America, Africa, Southeast Asia, the Pacific Islands, and some areas of the Caribbean.^[16] In the United States, dengue is most frequently found in territories such as Puerto Rico, the U.S. Virgin Islands, and American Samoa, though cases have also occurred in the continental United States, including Florida, Hawaii, Texas, Arizona, and California.^[16]

Certain groups face higher risk for serious illness from dengue. Children and elderly individuals are at greater risk for developing severe complications.^[2] The disease shows a seasonal pattern that corresponds with warmer, rainy months. In the southern hemisphere, most cases occur in the first half of the year, while in the northern hemisphere, the majority of cases appear in the second half.^[7] This timing aligns with increased mosquito breeding activity during warm and wet conditions.

Causes

Dengue fever is caused by infection with the dengue virus, which belongs to a family of viruses transmitted to humans through mosquito bites. There are four distinct but closely related types of dengue virus, designated as DENV-1, DENV-2, DENV-3, and DENV-4.^[4] Each type can cause dengue fever, and infection with one type does not protect against the others. In fact, having dengue once can make subsequent infections with a different type more dangerous.

The virus spreads to people primarily through the bite of infected female mosquitoes, particularly the Aedes aegypti species, though other Aedes mosquitoes such as Aedes albopictus can also transmit it.^[6] These mosquitoes become infected when they bite a person who already has the virus in their blood. The virus then multiplies inside the mosquito, and once infected, that mosquito can transmit the virus to other people for the rest of its life.^[16]

The transmission cycle works like this: an infected person carries the virus in their bloodstream, especially during the first week of illness when they have a fever. If a mosquito bites this person during that time, it picks up the virus. After the virus replicates inside the mosquito, that mosquito can then pass the virus to healthy people through subsequent bites.^[13] This is why it’s important for people with dengue to avoid mosquito bites—not just for their own comfort, but to prevent spreading the disease to others.

While mosquito bites are by far the most common route of transmission, there are rare exceptions. A pregnant woman infected with dengue can pass the virus to her baby during pregnancy or around the time of birth.^[2] In very uncommon cases, people have contracted dengue through blood transfusions, organ transplants, or accidental needlestick injuries in healthcare settings.^[16] However, dengue is not contagious from person to person through casual contact, unlike respiratory illnesses such as the flu.

Risk Factors

Several factors increase a person’s likelihood of contracting dengue fever or developing severe complications. The most significant risk factor is living in or traveling to tropical and subtropical regions where dengue is common. These areas provide ideal conditions for the Aedes mosquitoes that carry the virus—warm temperatures, standing water for breeding, and dense human populations.^[3]

Having had dengue before actually increases your risk of severe illness if you get infected again with a different strain of the virus. This might seem counterintuitive, but it relates to how the immune system responds. After recovering from one type of dengue virus, you gain lifelong immunity to that specific type. However, if you later encounter a different strain, your body’s antibodies recognize it as similar but not identical. Instead of destroying the new virus efficiently, these antibodies can actually help the virus enter your cells more easily through a process called antibody-dependent enhancement.^[2] This complex immune response explains why second dengue infections often cause more severe disease than the first.

Age plays a role in disease severity. Children and elderly individuals face higher risk for developing serious complications from dengue infection.^[2] Young children may be particularly vulnerable to severe dengue, sometimes called dengue hemorrhagic fever, which can lead to dangerous bleeding and shock.

Behavioral and environmental factors also matter. People who spend time outdoors during daylight hours in areas where dengue is present have greater exposure risk, since Aedes mosquitoes are most active during the day, particularly in early morning and late afternoon.^[8] Not using insect repellent, wearing clothing that leaves skin exposed, and living in homes without screens or air conditioning all increase the chance of being bitten by infected mosquitoes.

Certain occupations and activities carry higher risk. Travelers visiting endemic areas, especially those staying in urban or semi-urban regions, face increased exposure.^[3] People who work outdoors, community health workers visiting multiple homes, and those living in areas with poor sanitation or inadequate waste management—which creates breeding sites for mosquitoes—all have elevated risk of dengue infection.

Symptoms

Most people infected with dengue virus never develop any symptoms at all. For those who do become symptomatic, the illness typically begins four to ten days after being bitten by an infected mosquito.^[3] This waiting period, called the incubation period, can make it difficult to connect symptoms with a specific mosquito bite.

When symptoms appear, they often resemble a severe flu. The hallmark sign is a sudden high fever that can reach 104°F (40°C).^[2] This fever is typically accompanied by intense headache, particularly pain concentrated toward the front of the head. One distinctive feature of dengue is severe pain behind the eyes, which worsens with eye movement.^[3]

The muscle, bone, and joint pain associated with dengue can be so intense that the disease has earned the nickname “breakbone fever,” though it does not actually cause bones to break.^[16] This deep aching pain, combined with the high fever, can make simple movements extremely uncomfortable. Many patients also experience nausea and vomiting, which can lead to dehydration if fluids are not adequately replaced.

A skin rash commonly develops during dengue infection, often appearing a few days after the fever begins. The rash may initially show as flushing on the face, then progress to a more widespread red rash covering much of the body.^[3] Swollen lymph glands may also be noticeable. These symptoms typically last two to seven days, and most people recover within a week or two.^[3]

However, in about 1 in 20 people who develop symptoms, the disease progresses to severe dengue after the fever begins to subside.^[2] Warning signs of severe dengue usually appear 24 to 48 hours after the fever goes away, which is a critical period requiring close monitoring. These severe symptoms include persistent and intense abdominal pain, repeated vomiting (at least three times in 24 hours), bleeding from the nose or gums, vomiting blood, or seeing blood in stools.^[13]

Other alarming signs include rapid breathing, extreme fatigue, restlessness or irritability, pale and cold or clammy skin, and feeling very weak.^[3] These symptoms indicate that blood vessels are becoming damaged and leaky, and that the number of platelets—cells that help blood clot—is dropping dangerously low. This can lead to shock, severe bleeding, and organ damage. Severe dengue is a medical emergency requiring immediate hospitalization.

After recovering from dengue, many people continue to experience fatigue for several weeks.^[3] This lingering tiredness is common and typically resolves gradually as the body fully recovers from the infection.

Prevention

Since there is no specific cure for dengue fever, prevention focuses primarily on avoiding mosquito bites and reducing mosquito populations. The most effective personal protection involves using insect repellent containing DEET, picaridin, or oil of lemon eucalyptus on exposed skin, especially during daylight hours when dengue-carrying mosquitoes are most active.^[8]

Clothing choices matter for protection. Wearing long-sleeved shirts, long pants, socks, and closed shoes reduces the amount of skin available for mosquitoes to bite. Light-colored clothing may also be preferable, as mosquitoes are often attracted to dark colors. When spending time outdoors in areas where dengue is common, limiting outdoor activities during dawn and dusk—when mosquitoes are particularly active—can reduce exposure risk.^[8]

Creating physical barriers between yourself and mosquitoes is another important strategy. Using screens on windows and doors, sleeping under mosquito nets, and staying in air-conditioned spaces when possible all help prevent bites. For travelers to dengue-endemic areas, choosing accommodations with screened windows or air conditioning provides added protection.

Community-wide mosquito control is essential for dengue prevention. Aedes mosquitoes breed in standing water, so eliminating potential breeding sites significantly reduces mosquito populations. This means regularly emptying containers that collect water, such as flower pots, buckets, old tires, and water storage vessels. Even small amounts of water in bottle caps or discarded containers can serve as breeding grounds.^[3] Community members should change water in pet dishes, birdbaths, and vases at least once a week, and ensure that gutters drain properly without collecting water.

In some areas, public health programs conduct mosquito control activities including spraying insecticides and introducing biological control methods. Community participation in these efforts—such as allowing health workers to inspect homes for mosquito breeding sites—enhances their effectiveness.^[7]

A dengue vaccine exists in the United States, but its use is limited. The vaccine is recommended only for children aged 9 through 16 who have laboratory confirmation of a previous dengue infection and who live in areas where dengue is common.^[4] The vaccine is not approved for travelers who are visiting but not living in dengue-endemic areas. The manufacturer has discontinued production due to low demand, though doses will remain available in Puerto Rico until 2026.^[8]

Pathophysiology

Understanding what happens in the body during dengue infection helps explain why the disease can become severe in some cases. When an infected mosquito bites a person, it injects dengue virus into the skin along with its saliva. The virus then enters the bloodstream and infects various types of cells, particularly those of the immune system and cells lining blood vessels.

As the virus multiplies inside infected cells, the immune system responds by producing antibodies and activating immune cells to fight the infection. This immune response generates the symptoms people experience—the high fever, headache, and muscle pain are all part of the body’s attempt to eliminate the virus.^[14] The fever typically lasts for about a week as the immune system battles the infection.

In most cases, the immune system successfully clears the virus and the person recovers. However, in severe dengue, the disease enters a dangerous phase just as the fever subsides. During this critical phase, which usually occurs three to seven days after symptoms begin, the tiny blood vessels throughout the body become damaged and start to leak.^[7] This leakage allows fluid from the bloodstream to escape into surrounding tissues, causing swelling and reducing the volume of blood circulating in the vessels.

At the same time, the number of platelets in the blood drops significantly. Platelets are small cell fragments that clump together to form clots and stop bleeding. With fewer platelets available, the blood’s ability to clot is impaired, which can lead to bleeding from the gums, nose, or internally in the digestive tract or other organs.^[1]

The combination of blood vessel leakage and reduced blood volume can cause blood pressure to drop dangerously low, leading to shock. In this state, vital organs do not receive enough oxygen and nutrients, which can result in organ damage or failure. The accumulation of fluid in body cavities, such as around the lungs or in the abdomen, can cause breathing difficulties and other complications.^[7]

The mechanism behind why some dengue infections become severe while others remain mild is not fully understood, but the phenomenon of antibody-dependent enhancement plays a significant role. When someone gets infected with a second, different strain of dengue virus, antibodies from their first infection can recognize the new virus but cannot neutralize it effectively. Instead, these antibodies inadvertently help the virus enter cells more easily. The virus essentially hijacks these antibodies as a doorway into cells, allowing for greater viral replication and a more severe immune response.^[2]

This explains why second infections with dengue tend to be more dangerous than first infections. The immune system’s previous encounter with a different dengue strain, rather than providing protection, can actually worsen the outcome. This complex interaction between the virus and the immune system makes dengue particularly challenging to prevent and treat, and it complicates vaccine development efforts.

During the recovery phase, which begins after the critical period ends, the body gradually reabsorbs the leaked fluids and blood vessel integrity is restored. Platelet counts begin to rise again, and overall condition improves. However, this recovery process requires careful medical management in severe cases, as too much intravenous fluid given during treatment can lead to fluid overload once the blood vessels begin healing.^[7]