Because in this scenario we are the hosts.
Not you personally, you as in humans. Ok so imagine where you live is the pocket in question. You see if everyone where you live dies then sure you are right. Future generations will be lucky that the disease got less deadly due to selective pressure, but you are dead in this scenario. Thats bad.
lets assume for the sake of the argument that I understand the concepts you are talking about. I am not talking about the disease 20 years from now. I am not worried about catching the disease you are talking about. I am worried about the weird mutant disease that only lasts a month but manages to take out 50% of humans on the planet in that month.