The clinical signs of any disease are the result of an intricate relationship between the infectious agent (virus, bacteria, etc.), the host’s immune response, and the environment. Professional epidemiologists devote themselves to investigating the factors associated with the distribution, frequency, transmission and risk factors of diseases in populations. There are many viruses, bacteria, and parasites that can cause disease in swine. Over the years, pathogens have evolved to better infect and dodge the host’s immune system. The most dangerous diseases to consider are those diseases that can jump between species. The host is the animal that is exposed to and infected by the pathogen. Age is an important determinant because most diseases affect certain age groups, mostly due to the changing of the immune system that age brings. The host’s immune system is an important determinant of clinical disease. More susceptible populations are at a greater risk of disease outbreaks.