Natural balance, and the lack of it
Within an ecosystem, many microbes adapt to invade animals and plants, and animals and plants adapt to deal with these invading microbes. But where animals (including humans) and microbes have not evolved together, a microbial infection may run through a population like wildfire.
Nature has divided the world into countless areas, each with their own biotic (life-based) and abiotic (environment-based) factors. Together, these are called ‘ecological systems’ or ‘ecosystems’. Some ecosystems are huge and involve the entire planet and indeed beyond to the Sun and further still. Generally speaking, however, when we talk of ecosystems we are referring to a piece of our world where microbes, larger animals and plants, as well as the general environment have evolved together and reached an organized, workable, balance.
This balance is complex, but at its simplest one can think of it as where animals and plants adapt to cope with the local hot, cold, wet or dry environment; where microbes adapt to invade animals and plants; and where animals and plants adapt to deal with these invading microbes, and so on. The entire balance relies on interdependence. As the local areas change, so do the environments and the life-forms.
Different life-forms include different microbes. Microbes include eg viruses, bacteria, fungi, prions, and microparasites. Other invaders include ‘macroparasites’ like intestinal worms, lung worms, and many similar forms. Some of these microbes can be essential to the health of those animals and plants they invade, such as ‘good bacteria’ in a gut! Others cause disease, but only limited disease - ie the invaded animal or plant goes on living - a kind of mini ecosystem of its own.
Unfortunately, where animals (including humans) and microbes have not evolved together, differences in the animal’s or plant’s immune system and the microbes in that region are often significant - the balance isn’t right and the animal or plant lacks the right resistance. The otherwise normal presence of microbes and larger parasites in an animal or plant and the associated balance, or relatively limited disease, may go ‘out the window’ and instead result in major infection or infestation that uncharacteristically and virulently spreads through the population.