> If killing you means that the pathogen can spread more rapidly, it will evolve to do so. Sure, but killing the host is not generally the goal of a pathogen.

> If killing you means that the pathogen can spread more rapidly, it will evolve to do so. Sure, but killing the host is not generally the goal of a pathogen. Killing the host typically means the pathogen won't spread any further, and that direction leads to the pathogen's extinction. Or, the death of this family line, if you like. But, before worrying about that, if a virus does not address the immune system, it will not be able to spread at all - there will be no family line. The initial goal of a new virus is to evade the (highly advanced) immune system. Ebola apparently has a very naive way of bypassing the human immune system. That is, overwhelm the system, so that the host dies, but you have a small chance of spreading. This works, sometimes, for a little while, but not too much, maybe just enough to gain a few generations of evolution to try and improve your strategy. If you are already a latent virus in a related species, you might get a few attempts at this, as Ebola has. HIV does this in a frighteningly more effective way - a way which is more evolved - hide from the immune system while maintaining a route to spread to other hosts. This has to be a very subtle balance - hiding from the immune system is no mean feat. This is why I used HIV as an example - HIV somehow negotiates its own and the host's survival with the immune system (where Ebola has not been able to). The dance between immune system and virus ultimately leads to AIDS. Death of the host is not ideal for the local strain of HIV, but it had a good chance to reproduce in other hosts, so it's a good result for HIV as a whole. To become an effective and latent human virus, you need to evolve some sympathetic way of defeating the immune system that doesn't have a significantly negative result for the host. It's likely that HIV is still on that path, and Ebola might be on the start of that path. This is why we observe viruses to become less deadly and more easily spread over time. It's not necessarily true that this is the only evolutionary path, but the immune system presents such a huge barrier that you have to overcome this before meeting any secondary goals.