I don’t think that Anne Neville’s speculations are quite as far out in left field as Blake suggests. While there certainly are people in close contact with animals, both people and animals in both agricultural and residential contexts are healthier, and sanitation is significantly better, than that of two centuries ago. However, the notion that cross-species transmission has plateaued out has substanial merit. The documented pandemic plagues of Eurasia between roughly 500 BCE through the 17th Century CE mostly had their origins (insofar as we can tell) in viruses and bacteria that originated with or were carried by domesticated animals and exacerbated by high population density of concentrated agrarian and later preIndustrial societies combined with inadequate sanitation. Good sanitation played a part in reducing some epidemics, but many diseases resulted in an effect immediately evident to students of evolutionary theory; infectious organisms killed off those incapable of resisting their effects, leaving a population with a natural resistance to the worst effects, but capable of carrying the virus, while simultaneously excessively virulent strains destroyed their hosts while more moderate versions co-existed and thrived. The childhood illnesses of measles, mumps, and chickenpox (to name a few) are the fortunate successors, well adapted to living in human populations.
There were still, through the 20th Century, virulent infections, the worst of which were influenzas, but modern nutrition and sanitation, along with preventative public health measures limits the spread of such illnesses. Truely virulent viruses like those that cause Ebola, Lassa Fever, and other hemorrhagic fevers tend to kill off their victims too quickly to spread far. Also, having recently jumped from simian species to humans, they don’t tend to replicate effectively, and after several generations the shed viruses tend to be dysfunctional and incapable of sustaining infectious potential. It helps that they also originate in remote jungle environments with limited infrastructure and transportation and rarely spread beyond a few dozen miles before detection and quarantine, but even in an urban environment a non-airborne hemorrhagic fever would probably be limited in impact. (A truely aerosol hemorrhagic fever or a seriously virulent influenza, on the other hand, could wreck havoc.)
In any case, regardless of the infectious organism, 100% lethality combined with high likelyhood of transmission is unlikely, simply because it wouldn’t survive long enough to replicate and spread. Even bubonic plague has a mortality rate of less than 50% when untreated. The only infections that could potentially result in true population-threatening epidemics are probably those which routinely result in meningitis or encephalitis effects, owing to the difficulty of treating or ameliorating those symptoms.
William H. McNeill’s Plagues and Peoples goes into greater detail on the topic of codevelopment of infectious disease and the civilizations of Eurasia, and makes a good companion piece to Diamond’s Guns, Germs, and Steel.
Stranger