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The evolution of pathogens
The pathogens which cause infectious diseases in humans evolved over millions of years, establishing a symbiotic relationship with other living creatures as they, too, evolved. Some parasites are too small to be seen with the naked eye and are called microorganisms. They include bacteria, viruses and protozoa. Macroorganisms are visible and include helminths or worms, fleas, lice and ticks. *Homo sapiens sapiens evolved about 40,000 years ago and lived in small, isolated hunter-gatherer groups. They would have suffered from chronic infectious diseases such as schistosomiasis and amoebic dysentery which were debilitating but not bad enough to kill.
They may also have been infected by the parasites of wild animals such as the trypanosome which causes sleeping sickness, as well as whipworm, pinworm, hookworm and tapeworm from ingestion of uncooked or poorly cooked meat. Since humans and primates share much genetic material, including 99% blood compatibility, it is likely that they also shared many common parasites.
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