This sounds fascinating concept; equally impressive to grasp!
In an article published in scientific american, Humans Carry More Bacterial Cells than Human Ones, scientists claim human body to contain more bacterial cell than the human cell itself. So if you have 100 trillion cells in your body, about the same number of bacteria are are paying you homage. Nice host. Moreover, it has also been reported that they have also contributed to human genes (http://en.wikipedia.org/wiki/Human_Genome_Project, http://www.ornl.gov/sci/techresources/Human_Genome/home.shtml). Strangely, other species seem to have less connections with bacteria; or may be it is yet to be discovered.
By definition, Ecosystem is a functional unit consisting of living things in a given area, non-living chemical and physical factors of their environment, linked together through nutrient cycle and energy flow. Since they help to maintain various body processes, this makes human as a host and the body as an ecosystem.
We had already learnt that some bacteria were friendly and some were not. Identification of pathogenic bacteria and use of antibiotic treatment has been hailed as one of the great success in medical history. The side effects of antibiotics are not so unfamiliar and reasoned as killing off pathogenic as well as friendly bacteria. However, once we are able to understand the ecosystem of human body, curing "infectious" diseases should be just a treat load of another identified bacteria! Shall we call it Green Medicine?