Malaria is a serious and potentially fatal disease caused by parasites that infect mosquitoes, which then bite humans, thereby passing along the infection. Because malaria is a blood-borne illness, humans cannot pass it through casual or intimate contact.

About 2 billion people worldwide are at risk of malaria each year. This figure includes people who live in—and travel to—areas where mosquitoes spread the disease, typically in warmer regions of the world. According to the World Health Organization (WHO), an estimated 247 million people were infected with malaria in 2021. Of those, a little over 619,000 malaria-related deaths occurred, mostly in children.

In the United States, about 2,000 cases of malaria are diagnosed annually, most of which are acquired during travel to places where the disease is prevalent, such as parts of Asia and Africa, Central and South America, and the Caribbean and the Western Pacific. However, there have been recent reports of locally acquired cases in Florida, Texas, and Maryland. According to the Centers for Disease Control and Prevention (CDC), most malaria cases diagnosed in the U.S. are imported, usually by those who travel to countries where malaria is endemic. However, locally acquired mosquito-transmitted malaria cases can occur, even though the risk is extremely low.

Malaria may develop days or weeks after a person is bitten by an infected mosquito, or it may take months or even years for symptoms to arise. Travelers often return home well before they realize that they have the condition.

If left untreated, malaria may be fatal. In the U.S., 5 to 10 people die each year from malaria. However, treatments can typically cure the condition if patients begin taking medication in the early stages of disease. Complications and death may be more likely when patients don’t receive prompt treatment, or in those with other debilitating conditions.

People who are at greater risk of complications from malaria include pregnant women, children under age 5, travelers who do not normally live in regions where mosquitoes spread malaria, and those with immunocompromising conditions such as HIV/AIDS. (Older children and adults who live in regions where mosquitoes spread malaria develop partial immunity after being bitten many times, but this immunity is lost rapidly after moving to areas where malaria is not present.)

Malaria is an infectious disease caused by parasites that enter the body via mosquito bites. People with malaria typically experience fever, headache, nausea, chills, and fatigue.

There are more than 3,500 species of mosquitoes in the world, but only female mosquitos from the Anopheles genus can spread malaria parasites. First, the mosquito must bite a human with malaria, ingesting a small amount of their blood. The Plasmodium parasites that cause malaria live in the blood of a person with malaria, and are passed to the mosquito. Inside the mosquito, the parasites reproduce and spread to the mosquito’s salivary glands. The next time the mosquito bites, it transfers some parasites to the previously uninfected individual via its saliva, continuing the disease transmission cycle between mosquitos and humans.

Once the Plasmodium parasites are in the human body, they move to the liver and reproduce. Within several days to weeks—but sometimes up to a year or longer if the parasites remain dormant in the liver—the parasites leave enter the bloodstream, where they feed on hemoglobin and reproduce within individual red blood cells.

Eventually, the infected red blood cells rupture, enabling the parasites to spread to other red blood cells, where they continue to reproduce. Because malaria damages red blood cells, it may cause anemia in some people, which can be severe.

Malaria is caused by parasites that are transmitted to humans, in the overwhelming majority of cases, by mosquito bites.

The parasites that cause malaria in humans are:

• Plasmodium falciparum, a type most often associated with severe and life-threatening malaria
• Plasmodium vivax
• Plasmodium ovale
• Plasmodium malariae
• Plasmodium knowlesi, which can also cause severe disease, but is largely confined to areas in Southeast Asia

Because the malaria parasite is in the red blood cells of an infected person, it is possible for someone to get malaria in other ways, such as:

• A woman with malaria transmitting the infection to her fetus during pregnancy or delivery (in rare cases)
• Injecting oneself with a used needle that was shared by someone with malaria
• Through organ transplantation, if the organ is infected with malaria (in rare cases)
• Through a blood transfusion (in extremely rare cases)

People who have malaria may experience jaundice (yellowing of the skin and/or whites of the eyes) and a number of flu-like symptoms, including fever, chills, and fatigue. Some have cyclical bouts of symptoms that resolve within several hours, then recur.

Symptoms include:

• Fever
• Sweating
• Chills
• Headache
• Nausea
• Vomiting
• Diarrhea
• Anemia
• Muscle aches
• Fatigue
• Weakness
• Jaundice
• Enlarged spleen
• Enlarged liver
• Quickened breathing rate

Severe malaria infections may cause additional symptoms, including:

• Mental confusion
• Delirium
• Difficulty breathing
• Dark-colored urine
• Abnormal bleeding
• Low blood sugar
• Seizures
• Convulsions
• Kidney failure
• Shock
• Coma

Malaria during pregnancy may lead to complications for both the mother and fetus, including miscarriage, preterm birth, and low-birth-weight infants.

A patient may be diagnosed with malaria after sharing their medical history with a doctor, getting a physical exam, and undergoing a diagnostic blood test.

If you have traveled to a region where malaria is transmitted, you should share that information with your doctor, even if it was more than one year ago. Tell the doctor what symptoms you are experiencing and how long you have had them. The doctor will also want to know if you have HIV, sickle cell disease, or anemia, and/or if you are pregnant.

During a physical exam, doctors will look for possible signs of malaria, including jaundice. The doctor may feel your torso to see if your spleen or liver feels enlarged.

A doctor should offer blood tests to check for malaria. The condition is diagnosed when Plasmodium parasites appear in the blood.

Two main diagnostic blood tests are available, and doctors often offer both:

• A rapid blood test, in which a small amount of blood is placed on a test card; it will show the presence of proteins from Plasmodium parasites that are in the blood
• A blood sample is examined under a microscope; doctors look for the presence of Plasmodium parasites

If the initial blood tests come back negative, doctors may test the blood again for up to 3 times to look for the presence of parasites.

A variety of anti-parasite medications are available to treat malaria. Malaria remains completely curable if treated properly. The treatment varies, depending on factors such as:

• The Plasmodium species causing the infection
• The suspected geographic location of the mosquito bite
• The severity of symptoms
• Whether the Plasmodium species may be resistant to treatment, which varies by species and geographic location. This is of particular concern for malaria contracted in Southeast Asia, and concerns are now growing in Africa.

Patients diagnosed with malaria caused by Plasmodium falciparum are typically hospitalized until their symptoms improve because they are at risk of severe illness. They may be treated with one or more of the following medications:

• Artemether-lumefantrine (FDA-approved, and a preferred first-line treatment if available)
• Atovaquone-proguanil (also suitable, and should be used if artemether-lumefantrine is not readily available)
• Intravenous artesunate, for severe malaria (preferred if available)
• Chloroquine, which is only effective in parts of Central America, the Middle East, Haiti, and the Dominican Republic; in other regions, the Plasmodium falciparum parasite is resistant to this medication
• Quinine plus an antibiotic, such as doxycycline or clindamycin
• Mefloquine may be prescribed when other treatments aren’t effective, except in Southeast Asia, where the Plasmodium falciparum parasite is more likely to be resistant to the drug

Patients who are diagnosed with malaria caused by Plasmodium vivax or Plasmodium ovale need additional treatment, as they may remain dormant in the liver. These parasites may be treated with:

• Chloroquine (except in countries where Plasmodium vivax is resistant to treatment, including Papua New Guinea and Indonesia)
• Hydroxychloroquine
• Artemether-lumefantrine, atovaquone-proguanil, or quinine plus doxycycline or tetracycline, if parasites are resistant to chloroquine
• In addition, all individuals must be treated with primaquine or tafenoquine after being treated with other medications to prevent recurrences of malaria from dormant liver parasites; however, these medications should not be offered to patients with a G6PD enzyme deficiency, which must be determined by a blood test prior to giving these medications.

Patients diagnosed with malaria caused by Plasmodium malariae and Plasmodium knowlesi may be treated with:

• Chloroquine
• Hydroxychloroquine
• Quinine
• Artemether-lumefantrine
• Intravenous artesunate

Hospitalization is also important for monitoring symptoms and disease severity, and for supportive care, such as intravenous fluids.

Malaria can be life-threatening, so it’s essential for patients diagnosed with the disease to be treated immediately. Most people who are treated quickly recover within a few days. People who have malaria caused by Plasmodium falciparum may be ill for longer—four or five days after treatment rather than one or two days.

Severe malaria may cause complications, including seizures, fluid in the lungs, kidney failure, coma, or death. Even with proper treatment, up to 10% and 20% of people with severe malaria die. Death is of particular concern for those who develop cerebral malaria, which may cause altered mental status, seizures, and coma.

In pregnant women, malaria may cause miscarriage, stillbirth, premature labor, or low birth weight. About 50% of pregnant women with severe malaria die. Infants born to pregnant women with malaria also are at risk for developmental delays.

If not properly treated, some forms of malaria may recur over time, causing a patient to experience symptoms again, months or years after they have been treated. This is of particular concern for Plasmodium vivax and Plasmodium ovale.

Because malaria is potentially life-threatening, people traveling to a country where malaria is transmitted should visit their doctor four to six weeks before leaving to get a prescription for medication that can prevent malaria. They should also consider wearing long pants and long sleeves, applying DEET (an insect repellent) to clothing and exposed skin, and sleeping under mosquito nets to protect themselves from mosquito bites. (Mosquito nets that are treated with an insecticide are much more effective than untreated nets.) Women who may become pregnant also should consider postponing their travel. Vaccines are currently being tested and deployed in certain African countries, but none are currently available in the U.S.

“Yale Medicine benefits from a wealth of expertise in studying and treating malaria, both internationally and domestically,” says Yale infectious diseases specialist and malaria researcher Sunil Parikh, MD, MPH. “With over a half dozen physicians and scientists studying this disease from all aspects, including the parasite, mosquito, and human, travelers presenting with malaria will benefit from the latest knowledge and clinical care for this potentially devastating illness.”

Researchers from across Yale are actively studying various facets of malaria as part of MalarYale, a network that seeks to promote collaboration among malaria researchers at Yale and elsewhere.