Pediculus humanus capitis

The common head louse is only able to survive on humans. Many parasites are species specific – we often forget that when one creature goes extinct, so too do dozens of parasites which are species specific to it. Nevertheless, of all the many human-specific parasites, head lice are of interest as they are one of the few from which we benefit. A childhood infestation by head lice, although irritating, is a harmless affair. However, especially in places with poor sanitation, head lice boost natural immunity to the more dangerous body louse which transmits a number of harmful and potentially deadly diseases. Modern civilization has all but destroyed the need for the benefits of head lice, but those who live in poverty still benefit from this natural immunization. As a species, therefore, head lice still help us, and they in turn cannot survive without humans to live on. Although we may not have intended it, simply by continuing to exist we have allowed this species to survive, and even today people benefit from it.

Source: ListVerse

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