Yellow fever is a viral disease, found in tropical regions of Africa and the Americas. It principally affects humans and monkeys, and is transmitted via the bite of Aedes mosquitoes. It can produce devastating outbreaks, which can be prevented and controlled by mass vaccination campaigns. The first symptoms of the disease usually appear 3–6 days after infection. The first, or “acute”, phase is characterized by fever, muscle pain, headache, shivers, loss of appetite, nausea and vomiting. After 3–4 days, most patients improve and symptoms disappear. However, in a few cases, the disease enters a “toxic” phase: fever reappears, and the patient develops jaundice and sometimes bleeding, with blood appearing in the vomit (the typical "vomito negro"). About 50% of patients who enter the toxic phase die within 10–14 days. There is no specific treatment for yellow fever. Vaccination is highly recommended as a preventive measure for travellers to, and people living in, endemic countries.