>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.
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