This question comes up now and again. There was an excellent response the last time: http://www.reddit.com/r/AskHistorians/comments/1u7gwr/are_there_many_cases_of_a_food_taster_actually/

On a medical note, Salmonella would come from uncooked chicken and eggs and resistance would be unlikely. Food poisoning by preformed bacterial toxins in the food would take 4-6 hours to become symptomatic and resistance to a toxin is unlikely so most of the guests and testers would be affected around the same time. In these cases, most people ingesting these foods would be sick around the same time. Campylobacter jejuni from uncooked chicken could cause food poisoning and then 2-3 weeks later lead to Guillain-Barré syndrome (ascending paralysis) which likely would only affect one, if any, of the people who ate it.

On that note, it would be interesting to read over an actual case if someone could provide it.