The people of medieval Europe had very little understanding of the causes of disease, and limited sanitary practices. Because of this, Diseases such as influzena, tuberculosis, dystentery, and plague were common in the medieval period. In addition, even minor wounds would become infected and cause serious, frequently fatal, illness. Understanding of disease theory would not occur until well into the modern age.
Fortunately, Medieval disease and sanitation are not accurately portrayed in the SCA.