![]() These new acquisitions originate from evolutionarily distant host species. Most human pandemic infections were acquired horizontally very recently on the evolutionary timescale, even though diseases such as typhus, measles and smallpox first occurred in prehistoric times. Thus a shared habitat, rather than a shared ancestry, is important for the acquisition of many infections. When we domesticated ruminants, and animals such as dogs, cats and rats 'domesticated' us for the rich pickings around human habitation, we acquired many infections from our new neighbors. Further opportunities for horizontal crossover of microbes and parasites from animals to humans arose when humans spread out of Africa. Ashford argues that the great apes became more specialized forest dwellers at the same time that early hominids explored the savannah, and that human gut parasites resemble those of omnivorous baboons more than those of chimps because humans, like baboons but unlike chimps, are omnivorous. Human DNA might show 98% similarity to that of chimps, but we share less than 50% of our microbes and parasites with them. The majority of zoonoses, however, remain in their animal reservoirs and, so far as their sojourn in humans goes – even with limited human-to-human transfer (as with Ebola or severe acute respiratory syndrome (SARS)) – we can regard them as 'temporary exhibits'. They have diverged from their progenitors in the original host: for example, measles is now distinct from rindepest. The new acquisitions were initially derived from zoonotic infections but have flourished as self-sustaining infections in the human population. Writing on infections of humankind, Tony McMichael and I have called those that cospeciated with their hosts 'family heirlooms' and those that crossed over from other hosts in recent evolutionary time 'new acquisitions'. Grooming among monkeys and apes is not only a means of social bonding but also a useful way of controlling nits and lice (Figure (Figure2 2). ![]() Nits are the eggs of lice, expertly cemented onto hair shafts, as many parents know from painstakingly combing them out of their children's hair. We speak about feeling lousy and admonish our friends for nit-picking. For much of human history and prehistory blood-sucking lice have been so prevalent that they became part of our everyday language. ![]() Humans harbor three kinds of these ectoparasites: head lice, body lice and pubic lice. They are frequent parasites of birds and mammals, each host species having its own type of louse. Lice are small, wingless insects that cannot live independently from their hosts (Figure (Figure1). I shall examine this unfolding story in the context of what we know about microbial infections, and will look at the promiscuity of viruses through the lens of modern molecular technology and I will add my own speculation on why naked apes have pubic hair. It is a tale of infidelity that I shall begin with the recent research on lice of David Reed and colleagues and of Mark Stoneking's group who, on the basis of phylogenetic analysis, have speculated that we may have acquired a clade of head lice from another hominid species and pubic lice from gorillas they have also suggested that lice might help determine the date when humans adopted clothing. ![]() Here, I discuss how we may in the course of prehistory have acquired the lice, and how other infections may, like the typhus bacillus, come to be shared by us and the animal species with which we are in close contact. Written in 1934 and subtitled The biography of a bacillus, it tells the tale of that dreaded disease typhus, its reservoir in rats and its transmission among humans by lice. Hans Zinsser's Rats, Lice and History is a classic in microbiology.
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