Surely it is easier to spread through a living host?
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Killing is just an unintended side effect of them being to effective at using the hosts cells to reproduce.
Or the hosts immune response going overboard trying to fight back.
If the disease is highly contagious, then it doesn’t matter if the host dies in a few days, because during those few days most of the people living around the host would already get infected. If the disease isn’t too contagious, then it usually doesn’t kill or takes a relatively long time to kill (think about AIDS for example) for precisely this reason.
>Surely it is easier to spread through a living host?
It is!
That’s why contagious ailments that are truly and reliably deadly in humans are very very rare. The cases that do occur from time to time are **misunderstandings**.
The virus/parasite/microorganism might be perfectly adapted to not kill their host, their natural prey, but as it so happens in those cases *humans are not their natural prey*. Diseases become truly deadly if they did not originate in our own species but in domesticated or wild animals and by some quirk of mutation and lots of exposure managed to make the cross-species jump. The disease might be perfectly suited to not kill their *original* hosts, but can then (unintentionally) wreak absolute havoc in a human.
Viruses and parasites don’t “care” about their long term success, or more accurately, don’t have any ability to understand it. Each individual is just trying to make more of itself. If doing that causes collateral damage that ultimately snuffs out them in general because the host wasn’t able to pass them along to other hosts, the individual still was successful in making copies.
more often than not these pathogens are NOT trying ot kill their host. the fact they end up dead is a sideffect of how they operate, in fact some of the more dangerous pathogen either will actively not kill its host, or if that is unavoidable will be able to spread evne after the host expires.
for viruses this is because they are hijacking Cells to take over their replication systems ot multiply and will do so until the cell outright bursts with the copies. now this isnt an intentional thing its just that viruses are extremely simple structures that only have one goal and wont care bout anythnig else ot accomplish it : multiply.
for parasites the issue is that the parasite is leeching something the body needs to function, in order for them to grow and spread. for instance a tapeworm that gets itself in your GI will literally steal food from you , and if not for the fact this gets extremely painful later on it will happily kills its host by starving them, or perforating the GI tract ot infect other organs(most likely spilling the contents of the GI into the body killing you with Sepsis)
for some pathogens the issue isnt the pathogen itself even, but how the body is gonna rect to it. you can legitimately die from your own immune system gonig overboard.
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