Epidemics relating to the spread of infectious diseases are caused by the lack of knowledge on specific ways a germ is transmitted and the ability for treatments to be effective in controlling the spread of the disease.  There are viruses, bacteria, fungi, and protozoa  classifications of disease organisms.  Each strain can mutate when exposed in the human body or other living organisms and and form new strains of that disease. It is very difficult for scientists and doctors to keep up with this as many environmental factors can influence mutations also. So because of this, until new sanitation preventive measures and treatments are discovered, many diseases spread quickly resulting in an epidemic. Examples of these can be found on the link below the Center for Disease Control.