During the last 10,000 years humans became subject to ferocious evoloutionary pressures rising from infectious diseases. Up to Pasteur it was common that 50% of the children never reached the age five. Amsell Rothschild, the richest Jew in Frankfort ghetto, had 20 children but only ten survived. The poorer Jews may have had even higher infantile mortality rates. In the Jaszbereny Jewish cemetery I found many Kleins, most were the tombs of very young children I have never heard about. There are five classes of infectious diseases, possibly successive stages of transition through, from those found exclusively in animals to those that infect only humans:
Stage 1: pathogen is only present in animals and is not transmitted to humans
Stage 2: animal pathogen can be transmitted to humans but not between humans (e.g. rabies)
Stage 3: animal pathogen can be transmitted between humans for a few cycles, causing occasional outbreaks that die out quickly (e.g. Ebola)
Stage 4: animal pathogen can undergo more extended transmission between humans (e.g. yellow fever, dengue fever and cholera)
Stage 5: pathogens exclusive to humans that either co-evolved with us or made the animal-to-human jump (e.g. measles, mumps, rubella, smallpox, syphilis and HIV)
Most animal pathogens never pass Stage 1: adapting to an entirely different physiology is not an easy thing for a pathogen to do. But recently, we’ve been helping them make that leap through blood transfusions, international travel and intravenous drug use. People move pathogens around. The chances of a new zoonosis are very high, but till now it is being controlled successfully. But it is there, on the margins.