Contamination risk of bagged salads
It’s true that the risk of contamination for any given food goes up the more it is handled prior to packaging. Obviously, bagged salads are handled a lot more that whole heads of lettuce, and recent recalls on romaine have made consumers understandably wary of bagged salads.
But in reality, the risk of being sickened by bacteria on produce is still relatively low. If E. coli is present in a bagged salad, it was most likely also present on the whole head of lettuce used to make the salad. Bagged salads, however, have been washed up to three times prior to packaging, and one of those washes includes a chlorine solution to kill bacteria.
Why a triple wash? The reason is that salad greens are not flat and smooth but include grooves into which dirt and germs can fall. Typically greens are washed right after harvesting at the farm, and then are cleansed twice more during processing.