eli5: What is the goal of bacteria or viruses that make humans sick? Why do they weaken their host?
In: Biology
The goal for bacteria and viruses is to live, and like pretty much every living thing, spread their genes to create the next generation. A host’s immune system can sense an “invasion” (or, in the case of some ancient species that lack an advanced immune system, like horseshoe crabs, blood will clump around an infection instead of fighting with antibodies). Much of a host’s symptoms can be attributed to the energy it takes to ward off infection. What’s interesting is that viruses and bacteria really NEVER want to kill off their host. If the host is dead, the virus or bacteria can die, too.
It is like asking what goal humans have for climate change. They are trying to multiply and consumption/pollution has been a byproduct.
That’s not their goal. The only thing they want to do (and “want” is the wrong word, since they do not have any kind of thought process that leads them to want anything) is reproduce. That’s it. It’s an extremely powerful feedback loop – if a certain characteristic leads to increased reproduction, that characteristic is retained, enhanced and passed on. If it leads to less reproduction, that characteristic eventually disappears.
Also, the bacteria and viruses don’t actually cause fever or excess mucus production or other symptoms – it’s the body’s reaction to the invaders that produces these symptoms. There are countless bacteria and even small insects that co-exist peacefully in the human body – they live in our guts, bloodstream, hair follicles, etc., and the body does not go haywire in response. These organisms (and we humans) have evolved to “live together” so to speak. These organisms tend to be beneficial (like digestive system bacteria) or at least neutral (eyebrow mites), meaning they don’t consume so many resources from us that they threaten our ability to live and reproduce.
Combine these concepts and we get a system where the body “over-reacts” to invaders that haven’t yet “learned” how to live in us without causing a ruckus.
And then we have certain creatures that produce toxins as part of their existence (like botulism bacteria) but that’s an entirely separate discussion. Those creatures aren’t “trying” to live inside us – they’re perfectly “happy” elsewhere, but every now and then they get into our food supply and if conditions are right, produce a powerful toxin that greatly harms the body. But that’s not the same thing as a bacteria or virus that’s looking to use our bodies as a host.
They all “want” to live, divide and spread. They basically use your body as food and a living ground and a factory (viruses). Often the symptoms of a disease are just a side effect of them using your body and it fighting back (e.g. pain, fever), but sometimes the symptoms help spreading them (sneezing).
Evolution plays a big part here:
Over the long term, it is against the interest of pathogens (bacteria, viruses, etc.) to kill or damage its host body, since a dead human does not move around, limiting the chances to spread to other humans. We see that usually diseases become less damaging the longer they have infected humans (decades to millennia). AIDS viruses already have become less deadly. Ebola on the other hand is a very deadly disease, but quite rare, because people die before they can spread it a lot.
On the other side is the common cold. Theses viruses are so successful, because they only make you sneeze a lot, but leave you enough energy to still go everywhere. They are the most “successful” disease, and due to their adaptation to the body, only make you sick a little bit.
A lot of the deseases we have nowadays are so distructive because they were originally diseases that farm animals like cows, sheep, etc. were affected by. Because we used to live so close to these animals though the bacteria causing those diseases were able to infect us because of random mutations. So basically those diseases are more or less harmless for farm animals because the bacteria obviously only want a host for as long as possible… killing the host would also kill their home. But because humans dont have the neccessary physique to keep those bacteria in check inside their bodies the diseases are a lot more harmful.
This is btw also a reason why we europeans brought so many diseases to america when it was colonized but we didnt really bring any diseases back from there: there were barely domesticated animals in america. The only ones that the tribes living there could successfully domesticate were llamas. Which meant that the diseases that we europeans learned to deal with over centuries were completely new for the native tribes there and caught their immune system off guard, killing almost all of them.
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