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Extracts/Excerpt from Chapter 11 of Guns, Germs and Steel:

http://evomech5.blogspot.com/2007/01/guns-germs-and-steel-short-history-of.html

The major killers of humanity throughout our recent history - smallpox, flu, tuberculosis, malaria, plague, measles, and cholera - are infectious diseases that evolved from diseases of animals, even though most of the microbes responsible for our own epidemic illnesses are paradoxically now almost confined to humans.

Questions of the animal origins of human disease lie behind the broadest pattern of human history, and behind some of the most important issues in human health today. (Think of AIDS, an explosively spreading human disease that appears to have evolved from a virus resident in wild African monkeys.)

Microbes have evolved diverse ways of spreading from one person to another, and from animals to people ... Some microbes ... hitchhike [a ride] in the saliva of an insect that bites the host and flies off to find a new host. The free ride may be provided by mosquitoes, fleas, lice, or tsetse flies [or ticks] that spread malaria, plague, typhus, or sleeping sickness [or Lyme disease], respectively.

To sustain themselves [acute infectious diseases] need a human population that is sufficiently numerous, and sufficiently densely packed, that a numerous new crop of susceptible children is available for infection by the time the disease would otherwise be waning. Hence measles and similar diseases are also known as crowd diseases.

Crowd diseases could not sustain themselves in small bands of hunter-gatherers and slash-and-burn farmers ... [but] could have arisen only with the build-up of large, dense human populations. That build-up began with the rise of agriculture starting about 10,000 years ago and then accelerated with the rise of cities starting several thousand years ago. Among animals, too, epidemic diseases require large, dense populations and don't afflict just any animal: they're confined mainly to social animals providing the necessary large populations. Hence when we domesticated social animals, such as cows and pigs, they were already afflicted by epidemic diseases just waiting to be transferred to us. ... The close similarity of the measles virus to the rinderpest virus suggests that the latter transferred from cattle to humans and then evolved into the measles virus by changing its properties to adapt to us. ... Our intimacy with cattle has been going on for the 9,000 years since we domesticated them - ample time for the rinderpest virus to discover us nearby.