Blood transfusions — before blood types were discovered — were particularly dangerous, and unpredictable. If a patient received the wrong type of blood from a donor, the mixing of two incompatible blood types had a high risk of the blood clotting, severe immune reactions, and death. Such blood incompatibility caused destruction of red blood cells with symptoms such as fever, chills, muscle aches, nausea/vomiting, dark urine, shortness of breath, and rapid heart rate.