After three to four days most patients improve and their symptoms disappear. Fever reappears and several body systems are affected. The patient rapidly develops jaundice and complains of abdominal pain with vomiting. Once this happens, blood appears in the vomit and faeces. The remainder recover without significant organ damage.
Yellow fever is difficult to recognize, especially during the early stages.
A laboratory analysis is required to confirm a suspect case. Several other techniques are used to identify the virus itself in blood specimens or liver tissue collected after death. These tests require highly trained laboratory staff using specialized equipment and materials. This viral presence can amplify into regular epidemics. Until the start of this century, yellow fever outbreaks also occurred in Europe, the Caribbean islands and Central and North America.