Why would a germ want to make me sick?

November 10, 2007 6:35 PM

Why do bacteria and viruses make us sick?

I'm taking microbiology right now, and one thing is really confusing me: how does a host's illness benefit a parasitic organism? If a bacteria or a virus is primarily 'interested in' consuming and reproducing, wouldn't they be better served by hosts that are healthy instead of ill? I understand that some bacteria do have neutral or beneficial effects on their hosts, but I don't understand why so many organisms are pathogenic. It seems to me that especially organisms that cause fatal illnesses make little evolutionary sense. (Although I guess they do make sense since so many fatal pathogens are thriving!)

posted by serazin to Science & Nature (30 answers total)

As I understand it, illnesses tend to become less lethal precisely because the more deadly ones kill their hosts too quickly for them to spread widely enough. (Though this is apparently dependent upon transmission vector.)

There's a chapter on this in:

Evolution of Infectious Disease
By Paul W. Ewald

The beginning of the chapter is google-bookable.
posted by Comrade_robot at 6:39 PM on November 10, 2007

The ones that kill you fast are usually not adapted to your species, but infecting you only incidentally. The term is zoonotic diseases.

The ones well adapted to humans usually kill you much more slowly, giving the organism more chances to spread.
posted by NucleophilicAttack at 6:46 PM on November 10, 2007

Serazin, you're making a common mistake of assuming the evolution always produces perfect solutions. That isn't correct.

I think you're also making a conceptual mistake of looking at a single individual instead of at a population. If a disease can burn through a population, it becomes very successful, for a while anyway, even if it leaves a lot of corpses behind.
posted by Steven C. Den Beste at 6:49 PM on November 10, 2007

Just to expand on that, consider the "common cold". Like all viruses, you can only get a particular strain of common cold one time. After that, your immune system is sensitized to it and will prevent reinfection.

But the reason you get sniffly every winter is that the cold infects such a large number of people that it's agar dish is huge, and that means there's a much better chance, eventually, of helpful mutation. In particular, of mutations that change its antigen enough so that it is no longer recognizable as being the same virus as it was the last time you got infected, so it can infect you again.

It turns out that the common cold isn't very lethal, but the principle is the same. Influenza has been known to be very lethal, and it survives not by becoming a chronic infection, but by mutating and reinfecting the same prey animals over and over again. It's a different strategy -- and as human-target viruses go, influenza is one of the big success stories, despite the body count.
posted by Steven C. Den Beste at 6:53 PM on November 10, 2007

Response by poster: Steven C. Den Beste
I hear your point, but I guess because there are so darned many pathogens that make us sick, and so many of them seem to have evolved quite independently from each other, I get the sense that our illness is of some use for them. But ya, maybe its totally incidental, or it isn't really the best design for them, it just happened that way.

Comrade_robot:
checking out that book now!
posted by serazin at 6:55 PM on November 10, 2007

A lot of the "sickness" you feel during a cold or flu is actually caused by your own immune response, specifically by a protein called interferon. In fact, when interferon is administered as a drug, it causes flu-like symptoms in nearly everyone who takes it.

In the early days of a flu or cold infection, before interferon production ramps up, you feel fine even though are at your most infectious. It's only after your immune system starts churning out interferon (and downstream components of the immune response) that you actually feel sick. It's a little unfair to blame only the microbes for your fever, achy muscles, and fatigue.
posted by Quietgal at 6:56 PM on November 10, 2007

Response by poster: (I meant your first point, we were posting at the same time)
posted by serazin at 6:57 PM on November 10, 2007

Most of the symptoms of getting sick are from the immune system trying to fight the pathogen.
posted by aubilenon at 6:57 PM on November 10, 2007

I don't understand why so many organisms are pathogenic.

I don't think this is quite correct. There is an entire universe of microorganisms out there that are harmless to man, and would be unable to survive or reproduce in or on a person. The ones we all hear about--E. coli, Staph, Strep, Salmonella, etc--are part of that subset of microbes that can, under the right circumstances, survive in and cause disease to people.

And even those species are probably a long way from having evolved specifically to parasitize man. E. coli is a normal intestinal flora in cattle; it is only significant to us today as a human pathogen due to relatively modern food processing methods. Strains of it can be deadly not because they've perfected the art of parasitizing man, but because they produce some toxin that is harmless in cattle (or whatever) but just so happens to really muck with the human digestive tract. It would be like shoving a porcupine inside an elephant, and saying the resulting injury to the elephant was of some reproductive utility to the porcupine

Don't underestimate your immune system. It wipes out untold bazillions of pathogens all the time, without you ever being aware of their existence. Quite often the symptoms you notice are not those caused directly by the pathogen, but are your body's own efforts at expelling the bad thing in you.
posted by Brian James at 7:07 PM on November 10, 2007

Some factors to think about. First, in many cases the pathogenesis is related to transmission. Most diarrheal diseases reproduce in your gut, and the poop explosion is their escape and transmission mechanism. Respiratory infections need you to have respiratory symptoms for maximal transmission. Second, bugs normally have to deal with your immune system. For most infections, if they don't put the hurt on you, they're going to get cleared in a week without the chance to really spread. They have that week to maximally reproduce and get themselves out there, so they have to make a big show at it. Also from this aspect think of the many infections which are mostly pathogenic in weak people. In normals, they lie low, but when there's nothing to contain them they take over. Third, as others have said your real nasties are frequently zoonotic. They're a chronic endemic disease of whatever, and just happen to kill you. At the far end, tetanus, botulism, and gas gangrene are caused by soil anaerobes. They're built to fight and kill fungi; you're just in the way. Finally, there are lots of diseases which take a long time to die from or don't kill you flat out. Syphilis, mycobacteria (TB, leperousy) in the former and gonorrhea in the latter.
posted by a robot made out of meat at 7:08 PM on November 10, 2007 [1 favorite]

Two things:

You're making the mistake of thinking that evolution is directed and purposeful. It's not. When something works good enough for it not to help its behavior by changing slightly, then it probably won't make the last 1% to some Platonic ideal. There's enough selective noise and bountiful enough resources to make no real difference in competition between 99% "perfect" and 100% perfect.

Most "germs"--in fact, the vast majority--do not make you sick. You have about (on the order of) ten times as many "germs" in your body than you have human cells. In your perfect "healthy" state, you are a vast zoo of foreign critters. Now when one or two don't play nice and you get the sniffles, you happen to notice it. You have a terrible selective bias going on here.
posted by cmiller at 7:22 PM on November 10, 2007

Why We Get Sick by Ness and Williams. It's a book on Darwinian medicine, which answers some of your questions.

I study zoonotic diseases (H5N1), and I found this book to be useful in helping explain such questions to people who ask me about my research.
posted by carabiner at 7:25 PM on November 10, 2007

We're more pondering the theory of evolution than discussing microbiology. The earlier answer that evolution doesn't create 'perfection' seems to be the short answer. All that needs to happen for a life form to exist in the natural world is that it live long enough to reproduce.

Humans are in a debatable sense the most highly evolved life forms on Earth, yet we kill everything we eat. It would be kind of nice if we could eat a pig or an ear of corn and still have it around so we could eat it again later, but that apparently isn't how we evolved. We have done a pretty good job of making sure there's usually another ear of corn or another pig once we finish with the one we need now, but viruses aren't as organized as we are. And, why need they bother, there's no apparent shortage of humans or other hosts for them to move on to once they need another one, either.

References:
Life feeds on life - Disgustipated (Tool)
Mate, Spawn, and Die - Mate, Spawn, and Die (Lard)
posted by Bokononist at 7:26 PM on November 10, 2007

By the way, one of the things here is that when it comes to viruses, it's really hard to become chronic. Our immune systems are very good at clearing up virus infections. That's why so few of them have the ability to become chronic; it's a very specialized ability, and usually requires developing a way to cross the blood-brain barrier, because that's the only place that's safe from the immune system.

Herpes, papillomavirus, and just a few others have managed to develop this trick. For most viruses (e.g. influenza) it really isn't within evolutionary reach. So the way they become chronic is through antigen-mutation and regular reinfection, which is evolutionarily much easier.
posted by Steven C. Den Beste at 7:27 PM on November 10, 2007

Followup: I do believe peace, love and a respect of all life will eventually trump ruthless selfishness in the arena of survival of the fittest. We're just not there yet. Evolution is sloooow, and the question was kind of loaded for a jaded response.

OK Bye
posted by Bokononist at 7:36 PM on November 10, 2007

Dr Beste: chronic herpes lives in peripheral nerves (normally). HPV resides in skin tissue, not nerves at all. Your other major chronic viruses don't live in the brain either. Hep C lives in liver cells, HIV in helper T cells.
posted by a robot made out of meat at 7:46 PM on November 10, 2007

Robot: the "blood brain barrier" is a term that describes the barrier between the nervous system and the rest of the body; it isn't exclusively brain tissue.

I was under the impression that HPV does exactly the same trick as herpes of hiding in peripheral nerve ganglia between attacks, but I may have been wrong about that.
posted by Steven C. Den Beste at 7:57 PM on November 10, 2007

Micro was my favorite subject, but most courses are way skewed to the side of things that infect humans and make them sick. They really don't cover the things that infect humans and don't make them sick, like all this stuff here. Not to mention all the unclassified or undiscovered viruses that exist in healthy humans.

Also if you look through the list I linked to you'll see that there are a lot of things on that list that do make people sick, even though they are normally present on most people all the time. Most of the sickness caused by them results when they get somewhere they are not supposed to be or come in contact with an immunodeficient person. Essentially most of the illness caused by them is when they end up in the wrong environment, so they have already evolved to normally not cause people sickness.
posted by 517 at 8:07 PM on November 10, 2007

Dr Beste: No, it isn't. The BBB is only around the central nervous system. Some people talk about a blood-nerve barrier around myelinated peripheral nerves, but that is different in many ways. Notably, you can target analgesia at only peripheral nerves by using drugs that do not cross the BBB.

And yes, HPV really does just live in squamous epithelium.
posted by a robot made out of meat at 8:40 PM on November 10, 2007

It seems to me that especially organisms that cause fatal illnesses make little evolutionary sense.

Be careful to not anthropomorphize the germs.

A germ (bacteria, virus, prion, whatever) is interested in making, at minimum, just one more germ. Period. Once it has evolved a strategy to make sufficient germs that allows all germs to continue making germs within the host population forever, the germ is pretty much "done" when it comes to evolution.

Think about sharks. They swim, eat and make little sharks. Sure, tomorrow it might be "better" for them to evolve opposable thumbs and a larger brain, but who cares? They can already swim, eat and make little sharks right now. Done!

So, when it comes to germs ... they don't really give a rat's ass whether the host dies or not. Just as long as the host stays alive long enough to ensure that a sufficient number of little buddies are created.

Now think about Ebola. This virus usually leads to a raving, rampantly hemorrhaging human in a tribe/village/city spreading viruses to other humans in the tribe/village/city. From the perspective of the virus, this is a perfectly sweet little niche to settle in. Done!
posted by Cool Papa Bell at 8:46 PM on November 10, 2007

to echo Cool Papa Bell, bacteria and virii multiply at such an increased rate that many germ generations multiply and many opportunities for the germ to be transferred occur that the death of the host is far from an immediate concern for the bacteria or virus.
posted by kuujjuarapik at 9:20 PM on November 10, 2007

to answer the questions simply:

in reproducing, many bacteria produce waste. in a close environment, bacterial growth is limited by it's own toxic by products.

when, for example, some gets into your system, you bordy is reacting to the (often quite nasty) toxins produced by the bacteria. but, as others have noted, most symptoms are caused not by the bug, but by your body's response to it.
posted by Dillonlikescookies at 11:41 PM on November 10, 2007

er *closed environment (like a petri dish, for example)
posted by Dillonlikescookies at 11:42 PM on November 10, 2007

Why do bacteria and viruses make us sick?

Most of them don't. Your mouth and intestines are full of bacteria, and you are infected with many viruses that cause you no symptoms whatsoever.

Consider diarrhea - from cholera, say - one of the most common causes of death due to bacterial infection. Explosive, violent, voluminous diarrhea is a great way for bacteria to find another host to inhabit.

Bacteria and viruses don't know what they're doing. It's certainly true that the most lethal viruses - ebola, hantavirus, Lassa fever, the other hemorrhagic fevers - have not, historically, been the most dangerous to human populations. They kill so quickly that there is little time for the sick person to infect others.
posted by ikkyu2 at 12:11 AM on November 11, 2007

It's a very specialized ability, and usually requires developing a way to cross the blood-brain barrier, because that's the only place that's safe from the immune system.

The CNS (or PNS) is not an immunoprivileged zone. You just have to look at MS, GBS or JC virus infections to realise that. That being said, the Herpes members such as HSV and VZV are good at hiding out there, but for reasons much more complex than "the brain is safe from the immune system".

I don't think there's much more that needs to be added to the given answers except that a lot of pathogens need to cause some pathology to spread (rhinoviruses, influenza etc. are the obvious examples). And that some pathogens exist in other animals or environments besides humans - thus, there isn't a great deal of evolutionary pressure on the ones that cause severe disease in humans.

And I'll also add that HIV is an interesting case in an evolutionary sense: It has existed in the human population for a lot longer than the 30 odd years since we've known about it. It's believed, though, that because it infected individuals in remote areas of Africa (and therefore couldn't spread very easily) only slow-acting, less aggressive forms of the virus were able to persist. As it spread around the world, more aggressive types were able to spread without killing themselves out.
posted by kisch mokusch at 3:40 AM on November 11, 2007

Some pathogens do intentionally make you sick. They carry "virulence factors" on pathogenicity islands, or some of bacteria are themselves infected with phage which carry virulence genes for them.

In many cases, virulence factors are necessary to invade the host successfully by (say) breaking through mucosal barriers, and so on. Other factors make you intentionally sick (think of cholera toxin, which activates Gs of the large G protein family, ultimately resulting in massive diarrhea through constitutive activation of an ion channel.)
posted by NucleophilicAttack at 4:47 AM on November 11, 2007

Just remember: they were here first. Every "higher" organism is crawling with them all the way through. Even parasites have parasites; bacteria have viruses and viruses can scavenge one another's parts. But whether a given microorganism will make people sick comes down to a roll of the dice. They are just reproducing, and we are their jungles and their farms. Or we are junkyards and their factories, in the case of viruses. The cycle of transmission goes on, so long as the cycle of transmission goes on.

In other words, there are plenty of microbes that do meet their genetic ends, in the very way your question points out. They get into a host, kill the host, and, well, the end. Still, some of their brethren may get in and kill the host, but luck out (in a genetic sense, because the original infecting microbe is long 'dead') and get transmitted to other hosts anyway. The hosts that live longer can house and transmit more of the microbe. In this way, the microbes with the most efficiently lethal genes get filtered out, and you end up with microbes that only infect (and result in the death of) some of your cells, but not you, at least not immediately.

The majority of microbes, if able to survive in the harsh environment of the body and evade the immune system for a bit, are ultimately parasites that kill you only ever so slowly, if ever. And the majority of these were never killers to begin with, but survived long enough to multiply a whole bunch and get an immune response large enough for you to notice as "feeling sick." Some, as mentioned with Vibrio cholerae, are adapted to vertebrates or humans enough to have a highly specialized mechanism of transmission that incidentally involves the host being miserable and dying, but these are specialists that have the 'luxury' of killing hosts because of a steady, established cycle of transmission that has yet to be completely broken.
posted by zennie at 6:53 AM on November 11, 2007

Our immune systems allow bacteria to live in certain areas on our bodies without making us sick - in our gut, skin, oral cavity, etc. But if a bacteria can make it past those barriers, perhaps to the blood stream or inside the bladder, it's a like a ticket to an all you can eat buffet. No competition from other bacteria, and with the right virulence factors they can hang around and eat you up. What makes us sick is our immune response, which is trying to keep the bacteria from eating your entire bladder and it getting necrotic or whatever. Then the bacteria has to ramp up its virulence, and do things to confuse the immune system so it can keep eating and dividing. This is where stuff like secreted toxins come into play - most of them target immune cells. Also, the immune system isn't perfect, and under certain conditions it will over react and kill you, like in sepsis. So illness comes from two things: being eaten from the inside out; and inappropriate immune responses.
posted by fermezporte at 7:01 AM on November 11, 2007

Fermezport is pretty much spot on. A good instance of the immune system overreacting is in its response to hantavirus infection (some people might remember the outbreak in the Four Corners region back in the 90's). Hantaviruses code for about 4 proteins, none of which really does much to kill cells. The major symptoms in a hantavirus infection arise because the immune system really overreacts to the virus and just goes apeshit killing everything.

Also, Steven C. Den Beste, you are wrong about the blood-brain barrier keeping out the immune system. If it did keep out the immune system, your body would never be able to clean up any sort of viral encephalitis. It also wouldn't make sense to have cells presenting antigen on the other side of the barrier if T-cells couldn't make it across to have the antigen presented to them. The only places that the immune system cannot reach are the cornea (though I've seen at least one paper suggesting that may not be the case) and the testes (as some proteins in the testes are expressed well after initial immune self/non-self training during infancy).
posted by The Bishop of Turkey at 11:49 AM on November 15, 2007

The CNS has it's own immune cells. They are awesome little critters. Some sit with tendrils extended to monitor neighboring brain cells, and if something goes wrong they pull in their appendages and turn into macrophages. Seems the brain's innate immune system may also play a role in some neurodegenerative disorders. However, the non-CNS immune system does have a huge impact on the brain.
posted by zennie at 10:55 AM on November 18, 2007