Why Do Diseases Hurt Us?

It seems like the ideal way to be a parasite is to allow your host animals to proliferate and be well. The healthier the host, the more nourishment they can provide the parasite. So I have a few questions for those in the know:

Why do virii and bacteria harm us, when they’d be better off if we were healthy?

Are there any truly harmless parasites in the average human? The mites in eyelashes seem pretty chill.

As I think of it, some diseases can only spread if the host coughs, sneezes, or vomits, but is that the reason for all of it? A sexually transmitted disease that negatively affects one’s sexiness seems like it should be very unlikely.

To wax Agent Smith-ish for a moment, humans seem to be demolishing the planet on which we depend for life. It’s an instinctive race to grab the most resources that has produced economic and political systems that remove all guilt and forethought, do nothing but grease the slide into hell.

Are parasites doing the same? Does life clamor its way into these overly-successful dead ends every time? Is the cycle of mass extinctions a natural mirror of our boom-bust economics? I don’t know. But I do know this: Nature sucks. Nothing more to add, today.


  1. Emu Sam says

    If it’s harmless, it doesn’t meet the definition of parasite. Either one benefits without affecting the other (commensalist) or both benefit (mutualism). Humans are full of bacteria with which we have a mutualistic relationship. By some measures, the bacteria outweigh the human.

    Recent studies indicate mistletoe may be more mutualistic than parasitic with its oaks, exchanging energy and nutrients. Today I learned the word hemiparasite.

  2. suttkus says

    Amateur biology nerd here! I’m probably wrong a lot. But, since nobody else has chimed in, I’ll expound like I know what I’m talking about!

    Parasites tend to evolve in one of two directions, either toward causing less and less harm to the host, thus lowering their impact and giving themselves a long time live in the host and produce young, or toward producing the maximum number of offspring NOW and who cares what happens to the host. Both strategies have their advantages and are favored by different circumstances. For instance, if you’re a parasite that has no way to avoid detection by the host, you might as well maximize reproduction now because you aren’t going to be staying here long anyway. Forget the security deposit and trash the place!

    As for how many parasites we might have that aren’t causing us any harm, well, it’s a bit hard to say. We certainly know of some, but most of what we know about our epifauna (stuff that lives with us) comes from detecting them when they cause things to go wrong. A completely harmless parasite is one we’re unlikely to notice.

    Well, I said “parasite”, but harmless parasite is an oxymoron. Vocabulary time! The public uses the word “symbiosis” to mean two species that live together to mutual benefit. Scientists,however, use the word “symbiosis” to mean any close living arrangement between two different species. Symbioses are then divided up into three different types: mutualism (both species benefit from the arrangement), commensalism (one species benefits, but the other isn’t harmed), and parasitism (one species benefits to the detriment of the other). Unfortunately, biology is messy and none of these categories are quite cut and dried.

    You mentioned eyelash mites. There are actually two species that live on humans, both in the genus Demodex, but I’m too lazy to look up the species names. Despite the common name, they live in hair-follicles all over your body, but they they are most happy on your face. One species lives in your hair follicles proper, the other species lives in the oil glands attached to your hair follicles. How polite! They share! (Or maybe one of them considers themselves forced into low-rent ghettos, I don’t know!) By adulthood, most humans in the US (and, presumably, the world, though studies outside of western civilization are scant) are infected with them, and the vast majority of those humans will never notice them.

    But some will. Because under some circumstances, these normally commensal organisms begin causing damage. Several skin diseases are caused by overpopulation of Demodex mites. Sometimes these are caused by immune system malfunctions that let the mites breed out of control. Sometimes they seem to be caused by imbalances in our skin’s other symbiotic partners that might, somehow, serve to keep the Demodex in check. Mostly, we just don’t know.

    Of course, some parasites go far beyond just evolving to be commensal. It’s widely suspected that before our mitochondria were our happy little partners, they were a parasite invading our ancestors! They evolved from parasites, to commensals, to mutualist helpmates, and now we’re entirely dependent on them to such an extent that they’re just considered part of our bodies and not separate organisms at all! It’s thought that many of our organelles might have had similar origins. You’re a collection of parasites that decided to work together! With such a legacy of togetherness, why is it we still can’t get our act together as nations?

  3. robert79 says

    Bias — The ‘diseases’ that simply stay confined to a single individual and have no harmful effects whatsoever will never get diagnosed. We only notice the ones that go out of control.

  4. says

    Are parasites doing the same? Does life clamor its way into these overly-successful dead ends every time? Is the cycle of mass extinctions a natural mirror of our boom-bust economics?

    To some extent, yes. Some organisms are specialized to be short-lived, quickly reproducing and opportunistic. They seize on any available resource and gobble it up as quickly as possible, making lots of spores/eggs/whatever. These then wait around for the next resource to become available. It’s a fine strategy, if you don’t mind having a boom-bust population. And giving up on maintaining a large brain.

    The thing is, these organisms never last long anyway. It’s not worth the bother to tend to your environment if you’re not planning to stick around. I think parasites harm their hosts mostly because it would be too much work not to. The work you put into not killing your host is taking away from your ability to reproduce, so you get outcompeted.

  5. invivoMark says

    If it’s truly harmless, then it’s not a parasite. Duh.

    Less trivially though, there is a tendency for pathogens to lose their virulence the longer they are able to evolve along with a species. Take the herpesvirus family, for instance. Most human herpes viruses are highly species specific, and have evolved along with humans for probably millions of years. Other than simplex, most of them rarely cause disease. Epstein-Barr, Kaposi’s Sarcoma Herpesvirus, and cytomegalovirus only infect humans, have very high infection rates (up to 95% of adults for EBV), and only cause disease in a very small percentage of those infected. Herpes simplex, the most likely herpesvirus to cause disease, is also the only herpesvirus that infects a broad range of species, including mice and rabbits.

    Similarly, human papillomaviruses, which are mostly asymptomatic, are highly species specific. HIV is pathogenic because it recently jumped species, and hasn’t yet adapted to its human hosts. SIVs in nonhuman primates tend to be harmless and asymptomatic.

    There are a few exceptions to this generality, though. These generally involve organisms whose spread is highly dependent on something that’s harmful to the host. Most species of Salmonella infect a broad range of hosts, and in humans cause a bit of an upset tummy. The few strains of Salmonella that are more specific to humans are the ones that cause typhoid fever. They’ve evolved to be more pathogenic than their cousins because the more they can make their human poop, the more they get to spread to other humans.

    But these are exceptions. Generally, the more species specific the pathogen, and the longer it’s been with the same species, the less pathogenic it is. The ones that hurt us the most are the ones who just haven’t gotten used to us yet.

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