In chapter 11 of "Guns, Germs, and Steel," by Jared Diamond, the concept of germs being passed from animals to humans is discussed. Diamond describes the attack and growth of various pathogens suffered by the animal to human contact in the Agricultural societies. The Agricultural societies were better suited for these type of diseases, than the Hunter-Gather societies. Because the population, in the Agricultural societies, was so much larger than the Hunter-Gather communities the pathogens were more easily spread from person to person. Early on, the farmers even use to collect the feces of the humans and mix it into the soil for fertilizer. This also increased the spread of these diseases.
Diamond talks about four main stages of this animal to human infection. The first stage is the diseases that we simply pick up once and a while. My cousin just got out of the hospital a few weeks ago because of Cat-Scratch Fever. This is an example of that type of disease. In the second stage writes Diamond ;
.."a former animal pathogen evolves to the point where it does get transmitted directly between people and causes epidemics. These are diseases like Measles (came from cattle), Tuberculosis (cattle), Flu (pigs and birds) and Smallpox (cattle and livestock). A third stage in the evolution of our major diseases is represented by former animal pathogens that did establish themselves in humans that have not yet died out, and still may become killers of humanity. Lassa Fever is a good example of this type of pathogen. The fourth and final stage of this evolution is represented by the major, long-established epidemic diseases confined to humans. These diseases must have been the evolutionary survivors of far more pathogens that tried to make the jump to us from animals - and mostly failed." (pg. 207-209)