Always wash lettuce carefully, and even if you've done it, if you don't eat it at once, wash it yet again before eating it the next day. Cross contamination can take place at the grocery store when the produce is being readied for display.

Remember that residual bacteria can reestablish themselves on the surface even after washing. Cantaloupe has been contaminated when the cutting knife passes through the rind and into the flesh.

A little known fact is that the source of food poisoning in potato salad is not the mayonnaise, but the potato which is a very good medium for culturing bacteria. Of course, no one here eats potato salad anymore, right?