Yellow fever is a hemorrhagic fever caused by a virus spread by a particular species of mosquito. It is most common in areas of Africa and South America, affecting travelers to and residents of those areas.

In mild cases, yellow fever causes fever, headache, nausea and vomiting. But yellow fever can become more serious, causing bleeding (hemorrhaging), heart, liver and kidney problems. Up to 50 percent of those with the more severe form of yellow fever die of the disease.

There is no specific treatment for yellow fever. But getting a yellow fever vaccine before traveling to an area in which the virus is known to exist can protect you from the disease.

During the first three to six days after you've contracted yellow fever — the incubation period — you won't experience any signs or symptoms. After this, the virus enters an acute phase and then, in some cases, a toxic phase that can be life-threatening.

Acute phase
Once the yellow fever virus enters the acute phase, you may experience signs and symptoms including:

• Fever
• Headache
• Muscle aches, particularly in your back and knees
• Nausea, vomiting or both
• Loss of appetite
• Dizziness
• Red eyes, face or tongue

These signs and symptoms usually improve and are gone within three to four days.

Toxic phase
Although signs and symptoms may disappear for a day or two following the acute phase, 15 percent of those with acute yellow fever then enter a toxic phase. During the toxic phase, acute signs and symptoms return and more severe and life-threatening ones also appear. These can include:

• Yellowing of your skin and the whites of your eyes (jaundice)
• Abdominal pain and vomiting, sometimes blood
• Decreased urination
• Bleeding from the nose, mouth and eyes
• Heart dysfunction (arrhythmias)
• Liver and kidney failure
• Brain dysfunction including delirium, seizures and coma

About 20 percent to 50 percent of those who enter the toxic phase die of the disease. The rest usually recover without significant problems.

Yellow fever is caused by a virus that is spread by the Aedes aegypti mosquito. These mosquitoes thrive in and near human habitations where they breed in even the cleanest water. Most cases of yellow fever occur in sub-Saharan Africa and tropical South America.

Humans and monkeys are most commonly infected with the yellow fever virus. Mosquitoes transmit the virus back and forth between monkeys, humans or both. When a mosquito bites a human or monkey infected with yellow fever, the virus enters the mosquito's bloodstream and circulates before settling in the salivary glands. When the infected mosquito then bites another monkey or human, the virus then enters the host's bloodstream, where it may cause the serious illness.

Traveling to Africa or South America
Traveling to an area in which the yellow fever virus is known to be present puts you at risk of the disease. These areas include sub-Saharan Africa and tropical South America.

Even if there aren't current reports of infected humans in these areas, it doesn't mean you're risk-free. It's possible that local populations have been vaccinated and so are protected from the disease, or that cases of yellow fever just haven't been detected and officially reported.

If you're planning on traveling to these areas, you can protect yourself by getting a yellow fever vaccine at least 10 to 14 days prior to traveling.

Anyone can be infected with the yellow fever virus, but older adults are at greater risk of getting seriously ill.

See your doctor at least 10 to 14 days before traveling to an area in which yellow fever is known to occur, to see if you need vaccination.

If you have any signs or symptoms of yellow fever, see your doctor, and tell him or her if you've recently traveled to a region in which the disease is known to occur.

Diagnosing yellow fever based on signs and symptoms can be difficult because its early signs and symptoms can be easily confused with those of other diseases such as malaria, typhoid, dengue fever and other viral hemorrhagic fevers. To diagnose your condition, your doctor will likely ask about your medical and travel history and test your blood for evidence of the virus.

Medical history
Your doctor will likely ask about your medical and travel history and any exposure to mosquitoes. Be sure to describe international trips in detail, including the countries you visited and the dates, as well as any contact you may have had with mosquitoes.

Blood tests
Laboratory tests, usually using a sample of your blood, are needed to confirm a diagnosis of yellow fever. If you have yellow fever, your blood may reveal the virus itself. If not, blood tests known as enzyme-linked immunosorbent assay (ELISA) and polymerase chain reaction (PCR) also can detect antigens and antibodies specific to the virus. These tests may take several days.

Yellow fever results in death for 20 percent to 50 percent of those infected. This usually occurs within 10 to 14 days from the start of infection. Complications during the toxic phase of a yellow fever infection include kidney and liver failure, jaundice, delirium and coma.

Those who survive the infection recover gradually over a period of several weeks to months, usually without significant organ damage. During this time a person may experience fatigue and jaundice. Other complications include secondary bacterial infections, such as pneumonia, or blood infections.

No antiviral medications have proved helpful in treating yellow fever. As a result, treatment consists primarily of supportive care in a hospital. This includes providing fluids and oxygen, maintaining adequate blood pressure, replacing blood loss, providing dialysis for kidney failure and treating any other infections that develop. Some people receive transfusions of plasma to replace blood proteins that improve clotting.

If you have yellow fever, you may also be kept away from mosquitoes, to avoid transmitting the disease to others.

Vaccine
A safe and highly effective vaccine prevents yellow fever. Yellow fever is known to be present in sub-Saharan Africa and parts of South America. Talk to your doctor about whether you need the yellow fever vaccine at least 10 to 14 days before traveling to these areas or if you are a resident of one of them. Some of these countries require a valid certificate of immunization in order to enter the country.

A single dose of the vaccine provides protection for at least 10 years. Side effects of the yellow fever vaccine are usually mild, lasting five to 10 days, and may include headaches, low-grade fevers, muscle pain, fatigue, and soreness at the site of injection. More significant reactions — such as developing a syndrome similar to actual yellow fever, inflammation of the brain (encephalitis) or death — can occur, most often in infants and older adults. The vaccine is considered safest for those between the ages of 9 months and 60 years. Talk to your doctor about whether the vaccine is appropriate if your child is younger than 9 months or you're older than 60 years.

Mosquito protection
In addition to getting the vaccine, you can help protect yourself against yellow fever by protecting yourself against mosquitoes.

To reduce your exposure to mosquitoes:

• Avoid unnecessary outdoor activity when mosquitoes are most prevalent, such as at dawn, dusk and early evening.
• Wear long-sleeved shirts and long pants when you go into mosquito-infested areas.
• Stay in air-conditioned or well-screened housing.
• Apply permethrin-containing mosquito repellent to your clothing, shoes and camping gear and bed-netting. Use repellent with a 10 percent to 30 percent concentration of DEET on your skin. Choose the concentration based on the hours of protection you need — a 10 percent concentration of DEET is effective for about two hours, whereas higher concentrations last longer. Keep in mind that chemical repellants can be toxic, and use only the amount needed for the time you'll be outdoors. Don't use DEET on the hands of young children or on infants under 2 months of age. Instead, cover your infant's stroller or playpen with mosquito netting when outside. According to the Centers for Disease Control and Prevention, oil of lemon eucalyptus, a more natural product, offers the same protection as DEET when used in similar concentrations.