Err, your immune system can cope with a bit of bacteria. But if you don't wash your salad and get a massive load into yourself, your body will deal with it by extorting everything in your stomach. E.g. you'll puke the entire night. You're welcome.
There are customers visiting my company. I was washing my hand in the bathroom sink when one of them, after doing his business, put his left hand behind, opened the faucet with the right, wet his fingers, closed the faucet, and left. Disgusting piece of shit.
I understand the idea of removing the basic dirt and grim that could still be left on the surface of the lettuce. But the idea that running the vegetable under the water has any help in sterilizing it has to be pseudoscience. Too many adults have this mentality that washing produce purchased from the grocery store drastically reduces your chance of food born illness. If your food is contaminated with harmful microscopic organisms in a food outbreak. I doubt washing it is going to change much.