Are all bacteria harmful?
No, not by a long shot. As a matter of fact, most bacteria are not harmful and many are beneficial or even essential to our wellbeing. For example, bacteria are essential for making cheese and play a crucial role in our digestion. Indeed, each of us has 10 times more bacteria in our body then human cells (http://www.scientificamerican.com/article.cfm?id=ultimate-social-network-bacteria-protects-health; http://biology.about.com/cs/bacteriology/a/aa032504a.htm; http://www.britannica.com/EBchecked/topic/48203/bacteria#toc39333).
The main bacteria that can be found on chicken meat and may be harmful are called Salmonella and Campylobacter. But there are dozens of different strains (called serovars) of these bacteria and many are harmless (or non-pathogenic). For example Salmonella Sofia, which can be found on poultry meat, does not induce any harmful effect. On the other hand, Salmonella Typhimurium is often pathogenic.