The overwhelming majority of microorganisms do not in any way threaten human health. Moreover, half of all biomass on Earth is made up of microorganisms, while the percentage of biomass made up by mammals is only 15 percent. In this microbiology course, topics discussed include how and where they live, the structure of the organisms, and how they collect food and energy. In the Middle Ages, a physician would diagnose a patient by observing the patient's physique to determine the cause of the patient's symptoms. The writings of Dioscorides served as the basis for most herbal medicine practiced up until 1500. Avicenna was interested in human anatomy. Therefore he dissected a dead body to study it. William Harvey was the first person to define the concept of blood circulating throughout the body correctly. The Fabricated (written between 1514 and 1544) is the first scientific treatise to examine the anatomy of the human body. The assumption made by Galen that there were connections between the two ventricles was debunked by Andreas Vesalius. In 1852, William Henry Gray was awarded the prize for his research on the development of chicks.