Precisely which of those microbial genes individuals carry around varies greatly, according to the human microbiome project, a five-year, $173 million initiative led partly by Boston-area scientists, which published its findings Wednesday. In that diversity may lie the answers to questions about diseases that have not been completely explained by our own genes - for example, why some people, but not others, get inflammatory bowel disease, or become obese, or get sick from an infection. Eventually, the research may point to ways to manipulate microbial populations to treat or prevent diseases.