Steve, correct. The issue was with E. coli O157:H7-contaminated bagged prewashed lettuce purchased and consumed in Minnesota last year. 250,000 bags were recalled. according to the FDA this has happened 19 times and a total of 425 people have become seriously ill, two have died.
Obviously, millions eat prewashed lettuce products and have not had a problem but it is quite true that immune-compromised individuals (either due to chronic illness or disease or people recovering from illnesses or surgeries and on anti-biotics) and pregnant women should not eat uncooked produce. Though washing of unwashed produce is strongly recommended there is evidence to suggest that that might not do it--that there is or might be enough pathogenic bacteria either present residually or within the produce (because of the water used to irrigate) to cause serious problems for some individuals even after thorough washing. I know microbiologists who counsel washing prewashed produce and many who say don't bother--if the packer didn't get it clean with their much better equipment there's no way a consumer with a kitchen faucet is going to do better.
A critical issue with food items that aren't going to be cooked is preventing cross contamination. This has been a huge deal commercially because numerous cases of illness (E coli, Listeria, Norwalk virus, Hepatitus A et al.) have been traced to cross contamination. It's vital that washed produce and other ready-to-eat food not come in contact with pathogenic bacteria or viruses. Not placing these items on or in things that have not been properly washed is important as well as using only clean utensils and hands. In many places workers handling RTE food must wear gloves (but that guarantees nothing if the gloves have not been donned at the right time or properly changed).
Changes in agriculture, production, processing, shipping, and in bacteria and virus over the past many years is most likely why we have seen a gross increase in food borne illness over the time when most of us here were kids. We all remember a time when a rare burger was common in any restaurant. Due to the changes in animal processing there developed a problem with E. coli O157:H7; due to issues with run-off and irrigation this bacteria has become prevalent in soils where most of our food is grown. Due to run-off from there it has become a problem elsewhere--all this in the past 30 years--and that's talking about just one type of bacteria.
Anyway--Rita, I can't think of any other way that's better to prep salad ingredients ahead of time but a word of caution: I don't recommend doing too much too soon (spoilage issues) and it is important that the salad ingredients remain cold, especially because you'll be vacuuming. Wash in flowing water first, then chill in ice water, then spin and pack.