From what we've seen so far from the trailers, it looks like the ampules excrete some sort of shape-shifting substance. I'm thinking that the substance attaches to and bonds with human DNA (there is actually dialog in the trailer of someone screaming to "cut it off", I'm guessing that means cut off the infected parts of his body). The other crew members bring the infected people back to the ship and they either turn into Xenomorphs or Space Jockeys (the human form, not the one with the elephant suits). This fits in really well with the themes of Ridley's previous film, Alien, where it's what's inside the body that is truly terrifying.
EDIT: There are also scenes in the trailers where black spots appear on humans, I'm assuming these are the signs/symptoms of infection.