With the exception of some foods such as sterilised foods in canned produce, all foods contain bacteria. Some of these bacteria are harmless and occur naturally as part of the normal microflora of the chicken gut. There are some strains however that can cause illness in humans such as salmonella and campylobacter.
There are a range of biosecurity measures implemented on chicken broiler farms to reduce the risk of introducing foodborne pathogens. For example, water supplies are treated to kill bacteria in the drinking water.