This video shows off one randomized mission type: Blood Infection.
A host has been infected with multiple pathogens, and your objective is to clear out as many as you can. Along the way, you will encounter malfunctioning blood valves, crystallized cholesterol, and genetically-modified enemies. Collecting anti-septic weapons and ability-boosting plasmids, the survival of the host depend on your success!
I'm hoping this gives a good idea of what the full game will consist of. Each area has randomized layouts and objectives, so every host's body should feel different.