context: Yellow fever is a viral infection transmitted by a bite from infected mosquitoes most commonly found in parts of South America and Africa. When transmitted to humans, the yellow fever virus can damage the liver and other internal organs and be potentially fatal. The World Health Organization estimates there are 200,000 cases of yellow fever worldwide each year, resulting in 30,000 deaths. Yellow fever appears to be on the rise internationally, due to a decreased immunity to infection among local populations, deforestation, climate change, and high-density urbanization. The CDC has identified 44 counties with a risk of yellow fever transmission, many of them with tropical climates. While the actual number of yellow fever cases among U. S. and European travelers to these at-risk countries is low, vaccination is advised for most international travelers to these countries, because yellow fever has no cure and can be deadly. Yellow fever is typically spread to humans from bites by infected mosquitoes. People cannot spread yellow fever among themselves through casual contact, although the infection can be transmitted directly into the blood through contaminated needles. A few different species of mosquitoes transmit the yellow fever virus; some breed in urban areas, others in jungles. Mosquitoes that breed in the jungle also transmit yelllow fever to monkeys, who, in addition to humans, are a host for the disease. Yellow fever gets its name from two of its most obvious symptoms: fever and yellowing of the skin. The yellowing occurs because the disease causes liver damage, hepatitis. For some people, yellow fever has no initial symptoms, while for others, the first symptoms appear from three to six days after exposure to the virus from a mosquito bite. An infection with yellow fever typically has three phases. The first phase of symptoms can last for three to four days and then, for most people, disappears. The first phase is generally non-specific and cannot be distinguished from other viral infections. The initial symptoms of yellow fever are: Fever and chills Flu-like symptoms such as muscle aches, headache, and vomiting The next phase is remission, which lasts for 48 hours. Patients improve. The majority recover. Unfortunately, a third, more toxic phase of infection occurs for 15% to 25% of patients. Ultimately, a condition called viral hemorrhagic fever can develop, with internal bleeding (hemorrhaging), high fever, and damage to the liver, kidneys, and circulatory system. The World Health Organization estimates that up to 50% of people worldwide who reach this severe phase of infection die, while half recover. The third-phase symptoms of yellow fever can include: Jaundice (liver damage), which causes yellowing of the skin and eyes Hepatitis (inflammation of the liver) Internal bleeding (hemorrhaging) Vomiting blood Shock Multisystem organ failure leading to death Yellow fever is diagnosed by your symptoms, recent travel activity, and blood tests. Yellow fever symptoms can mimic symptoms of other tropical disease such as malaria and typhoid, so call your doctor if you have symptoms of yellow fever and have recently traveled to a high-risk country. Because there is no cure for the viral infection itself, medical treatment of yellow fever focuses on easing symptoms such as fever, muscle pain, and dehydration. Because of the risk of internal bleeding, avoid aspirin and other nonsteroidal anti-inflammatory drugs if you suspect you have yellow fever. Hospitalization is often needed. Because there is no cure for yellow fever, prevention is critical. The yellow fever vaccine is advised for adults and children over age 9 months who are traveling to or living in countries with a known risk of yellow fever. Certain countries in Africa and Latin America with the highest risk of exposure to yellow fever now require proof of yellow fever vaccination before allowing you to travel there. Travel medicine clinics and state or local health departments usually offer the vaccine, which needs to be repeated every 10 years for people traveling to high-risk areas. These approved vaccination centers can also provide you with the International Certificate of Vaccination that you'll need to enter certain at-risk countries. Call your doctor right away if you develop a fever, flu-like symptoms, or other unusual signs after taking the vaccine. The yellow fever vaccine, in a few rare cases, has caused an allergic reaction, nervous system reaction, and life-threatening illness. Yellow fever vaccination is not advised for everyone. The vaccine can cause serious adverse effects in certain people. Efforts are under way to develop a killed vaccine that will be safer. Talk with your doctor before getting the vaccine if you: Have a compromised immune system, such as from HIV Have cancer or thymus gland problems Have had treatment that can disrupt the immune system, such as steroids or cancer treatment Have had a life-threatening allergic reaction to eggs, chicken, gelatin, or past yellow fever vaccine Are pregnant or breastfeeding Are age 65 or older Your child is younger than 9 months old. Keep in mind that vaccination has two goals: to protect the health of individual travelers coming into high-risk regions and to protect the public health of countries by preventing the import of yellow fever into their region. If you're exempt from vaccination for medical reasons, you may need to provide proof of exemption for entry into some countries. Vaccination is the most important measure you should take when traveling to areas where exposure to the yellow fever virus is possible. No other measure is more effective, but there are other valuable recommendations. You should: Use the right insect repellent for mosquitoes on exposed skin and follow package directions. Buy one with DEET, picaridin, oil of lemon eucalyptus, or an ingredient called IR3535. Cover your arms, hands, legs, and head to protect yourself from bites. Use a mosquito repellent containing permethrin on the outside of clothing, mosquito netting, and other gear. Use screens on windows and doors, and mosquito netting over beds, to keep mosquitoes away. Avoid the outdoors during peak mosquito hours (dusk to dawn for many types of mosquitoes that carry the infection). Keep accurate records of international travel dates, locations, and outdoor activities in case you need to identify a viral infection when you return.