The number of bacteria living within the body of the average healthy adult human are estimated to outnumber human cells 10 to 1.
Humans are living Microbiomes (ecosystems). Imbalances in this system can be caused via poor diet, lack of exercise, poor environment, use of antibiotics or countless other factors.
The linked article above talks specifically about inflammation, something caused to a large extent by diet. What inflamation does is decrease the number of competing bacterial species within our body, allowing few (and in many cases, harmful) bacteria to proliferate, wreaking havoc on our bodies and leading to cancer.