One of the things I mention to my students from time to time is that we as human beings are really an assemblage of different species living with each other. An interesting article by Nicholas Wade on some of the important parasites that live in and one us drives this point home. As the article notes, we have a whole zoo of wee beasties living in or on us.
Image courtesy of ARS
Certain bacterial parasites such as Streptococcus mutans which causes tooth decay and Helicobactor pylori, are not easily transmitted. This allows us to gain unexpected insight into the origin of these parasites. For instance, S. mutans is transmitted from mother to child and the distribution of different strains seems to parallel human migration patterns.
Helicobactor pylori from http://en.wikipedia.org/wiki/Image:EMpylori.jpg
One particularly interesting conclusion about parasites turns our understanding of tapeworms on its head. The life cycle of some these parasites alternates between humans and pigs and it had long been thought that pigs were the original source of the parasite. But tapeworm infections appear to predate agriculture or domestication of pigs suggesting to some researchers that pigs were initially infected by humans. Indeed one of the tapeworms we passed to pigs may have come from humans infected eating dogs or each other. So Wade concludes by asking which animal would be considered unclean by pigs if pigs had religion.
In case you're itching for more parasite stuff, Wade has an interesting summary of research into the evolution of lice at his blog, the Loom.
Crab Louse image courtesy of NY Dept of Public health
Out of Africa: The Origins of the Tapeworm