system
Given the following post, respond as if you're commenting on Reddit:
human
How can certain bacteria and viruses be deadly to one species but completely harmless to another?

Edit:  Wow. Thank you all for the great answers!
gpt
You could potentially get many answers to this question! One example is if the pathogen's surface proteins recognizing specific cellular receptors that exist on one species but not the other. If they cannot recognize and attach to the cell, they are not infectious. Another variation is the immunity of the species and whether it can keep the pathogen in check. For example, bats have sophisticated anti-viral mechanisms that keep them safe from otherwise deadly viruses (like SARS!).

Yet another variation is the part of the world the host lives in; many pathogens thrive in a specific environment, so it makes sense that they would adapt to recognize hosts that live in a similar environment.

The short answer to your question is evolution. The host and pathogen co-evolve together which reinforces their relationship. Occasionally, however, the pathogen can mutate and cross species, like what happened with SIV and HIV.