Apple cider vinegar offers strong antibacterial and anti-fungal properties. It can prevent the growth of nasty stuff in the digestive tract. Results of a 2005 study suggested that including ACV (or lemon juice) in foods could combat the presence of vomit-inducing bacteria like E. coli and Salmonella.
Given that lettuce is one of the major foods affected by recalls for food poisoning risks, why not make your own salad dressing that contains some ACV and lemon? Otherwise, just take your ACV like you would after any meal. This will reduce the pH level of your stomach and help kill any bad microbes that got through.