Just witnessed the most disgusting thing I've ever seen on a cruise so far

I recently got home from sailing Royal Carribean's Navigator of the Seas out of LA and I saw what I consider to be the nastiest thing I've seen to date on a cruise ship. I watched a man in front of me wipe his nose and then pick up the salad dressing container at the buffet, cover the open pour spout with his finger, vigorously shake the salad dressing, and then set it back, apparently deciding he didn't want any after contaminating it. I stood there staring with my mouth agape before notifying staff to remove the container of dressing.

How are people this disgusting?

I realized that had I not witnessed it, nobody would have seen/known about the contamination, which got me thinking: how often have I unknowingly eaten contaminated food at the buffet? I wish Royal would move to having the crew serve food at the buffet.