You’re confusing a few things together. Parasites usually depend on their specific hosts to survive, so they try to not kill them (or at least just a few of them), especially if they’re adapted. If they jump host to another species, the results may not be as good, and the new host may not be able to adapt and suffer more than the original host.
Commensals are specific in that both host and organism tend to benefit, or at least one does not harm the other. This is the case of some of the bacteria present in your GI or even respiratory tract. In appropriate numbers, many species don’t harm you, but if some specific species overpopulate, you may have problems. There are also barriers that prevent some bacteria to cause damage, but if they’re damaged by something else, then some germs can invade places they shouldn’t. For example skin bacteria penetrating into the mucosa or under the skin, or a respiratory system damaged by smoking or other irritant making overpopulation and migration of usually non-damaging bacteria into parts they shoudn’t (and the body being unable to escort them out).
Also, there are many germs in which animals or humans are not necessarily required for their replication, our bodies are just the accidental environment they encountered. Some types of bacteria or fungus exist happily in the soil, and can remain there, either dormant or replicating along happily. But if we accidentally get them in our skin or inside us, they can cause damage. Either because they produce things that affects our bodies, or because our immune systems react so strongly to them that it affects our bodies.
Curiously :dubious: I was just reading about much this very subject in “Perils Of A Restless Planet”.
Being a virulent microbe that kills many/most of its hosts can work out fine, if (for example) it’s cholera and the epidemic involves a city, so that its victims infect others before rapid death occurs.
Organisms that jump species can have an especially virulent course. Plague apparently cycled back and forth between rats and fleas for a very long time, then when infecting humans produced disastrous epidemics, before retreating into a less serious form and hanging around in a limited reservoir of animals.
Generally it does seem to be advantageous in terms of their long-range survival for microbes to colonize their hosts and exist either in equilibrium with them or to slowly cause detrimental effects.
Nature just doesn’t follow such an orderly, logical pattern much of the time.
Even when a parasite intends to kill its host as part of its lifecycle, parasitizing the wrong species turns out to be a bad decision.
<Rod Serling>Consider if you will Hydatid disease, AKA Echinococcosis.</Rod Serling>.
The parasitic tapeworm invades a prey animal’s body and forms large fluid-filled cysts… sometimes hundreds of them… in various body cavities and tissues. Each cyst contains one tapeworm which matures slowly. The host of this phase of the is considered the intermediate host, in that it can be almost any species that can consume vegetation or drink water infected with the eggs of the worm.
In this phase of the worm’s life, it must kill its host to enable the next phase of its lifecycle. After this, the dead intermediate host is eaten, cysts and all, by a carnivorous scavenger which is its definite host. This results in live tapeworms (from the cysts) entering the definite host’s intestines and doing the typical tapeworm “feeding off of intestinal bloodflow and laying eggs in passing poop” thing.
The intermediate host phase is where humans often get involved. If a human consumes echinococcus eggs, cysts will grow in the human’s body , sometimes to the extent of threatening the host’s life. But if a human dies, we generally don’t leave it out for scavengers, so this batch of parasites’ life cycle ends there. Bad choice, tapeworm. Dead-end intermediate host.
BTW, if you need some borderline nightmare fuel, search Youtube for “hydatid” and watch the surgical extractions of the cysts from people. (Example: baseball-sized cyst extracted from a brain! :eek:)