Kill Your Pathogens

Everything has a niche. The human niche is particularly broad, but even we have our limits. Some places and conditions push us beyond what we can endure—extreme hot or cold, insufficient water or food, lack of particular nutrients even when other nutrients are in abundance. But we are remarkably resilient.

Those things that would make us sick almost always have narrower tolerances than we do ourselves. Those “things that would make us sick” are parasites and pathogens—two terms that have some overlap. They’re all freeloaders. Parasites are organisms that take resources from the host, but don’t necessarily directly cause disease. Parasites come in two broad forms: ectoparasites, which remain on the outside of the body, such as ticks, fleas and mosquitos; and endoparasites, which set up shop inside the body of their hosts. At the point that a parasite causes disease in its host, it may also be referred to as being pathogenic. Pathogens are often organisms (e.g. bacteria, protozoans, or fungi), but can also be other kinds of entities, such as viruses or prions. Pathogens inherently cause disease in their host.

The endoparasites and pathogens that can withstand the ecological conditions inside of a human body are but a subset of all the parasites and pathogens in the world—a subset that can live under our conditions.

As humans we are capable of inhabiting a huge range of environmental conditions, but our internal condition remains remarkably stable. Consider temperature. Most animals generate their heat from the environment, and as such, their body temperatures fluctuate throughout the day. When a lizard is sunning himself on a rock, his body temperature is elevated, and his physiological processes run both hotter and faster than at night, when he has little source of heat.

In comparison, mammals, like birds, are both endotherms and homeotherms: rather than seeking heat from the external environment, we generate it internally (endo thermy), and we maintain a constant body temperature (homeo thermy). As such, species tend to thrive in fairly narrow internal temperature ranges. Downstream of that: the same is true of our freeloaders. Whatever environment the host provides is the environment in which the parasite lives and is adapted to. Whereas the internal parasite of a lizard may need to endure a large temperature range on any given day (depending on where the lizard lives), the internal parasite of a mammal, such as yourself, ...