Yellow fever, also known as Yellow Jack and other names, is an acute viral hemorrhagic disease. The virus is a 40 to 50 nm enveloped positive sense RNA virus, the first human virus discovered and the namesake of the Flavivirus genus. In high-risk areas where vaccination coverage is low, prompt recognition and control of outbreaks through immunization is critical to prevent epidemics. The disease may be difficult to distinguish from other illnesses, especially in the early stages.