The ampules start to leak

http://www.prometheus-movie.com/media/prometheusE_screencap8.jpg The first image shows what looks to be the first trip into the temple, It is interesting in it self as it shows the back of the giant head. Note the ground and how dry it is. the ampules are dry as well. http://www.prometheus-movie.com/media/prometheusO_screencap4.jpg Now take a look at the image above. I am guessing that this is Milburn about to meet the cobra huggar, but take a look at the surroundings. It looks to me that this is the base of the giant head in the same chamber. The architecture in the room is the same as the prior picture. Now look at the ampules they are all overflowing with the black goo. Somebody touched or did something on the first trip to activate the ampules. I think the cause of this reaction is something Holloway does and it is how he gets infected. We see Holloway in this image looking at the what i think is black goo in a bowl. http://www.prometheus-movie.com/media/prometheus_screencap35.jpg Then we cut to the next image of a close up of a goo between two fingers. http://www.prometheus-movie.com/media/prometheus_screencap35.jpg I think this is Hollowy interacting with the goo, it creates a slow infection and starts a transfer of material to the goo and in turn starts the engine of the ampules.