So lets say your species never bothers to make a cure or something to resist the disease could the disease eventually kill your whole entire race off? Or would it be like in the Dark Ages of our past where there were some people who were natrually resistant to the Black Plague? (My own achievment i made, made me curious about this)
If I remember correctly, they are planning on adding a "game-ending disasters" mechanic, including diseases, but you can turn that off, as they are also planning on being able to rewind a stage or more because of that exact thing happening. (also, I didn't know some people were immune... huh)
It's very hard for diseases to kill everyone because they rely on spreading person to person. So say a disease kills 90% of people then there are so few people who have so little contact that the disease can no longer spread.
Successful diseases are things like the common cold because it doesn't kill you and that means you will spread it to others.
The most successful bacteria which infect humans are those that live in your gut because they are in a symbiotic relationship with you and you look after them indefinitely.
In general it's often in a diseases best interests to be mild.