The microbiome is all of the microorganisms (bacteria, viruses, parasites, etc.) that live in and on our bodies. These microbes help us digest food and improve our immune system. However, when the microbiome is in an irregular state, or “dysbiosis”, it can become harmful to our health. In fact, dysbiosis often accompanies disease. Several diseases are discussed in my thesis with chapters focusing on breast cancer and multiple sclerosis. Additionally, I present a new way to analyze the microbiome and then reveal how it could be used to predict treatment effectiveness based on the microbiome. One of the ways the microbiome impacts our health is through its production of small chemicals, or metabolites, which can influence normal functioning of our bodies. Thus, another chapter discusses how metabolites in the blood are altered after dietary supplementation. The final chapter presents the latest work, an investigation of both the oral microbiome and metabolome compositions in people with multiple sclerosis (pwMS). Many prior studies have found dysbiosis in the gut of pwMS, but few studies have looked at the oral microbiome. Additionally, in patients with other neurological diseases, their oral metabolites tend to be altered, however no study on the oral metabolites of pwMS has been undertaken. I ultimately found alterations in both the oral microbiome and metabolome of pwMS compared to healthy individuals. These findings provide us with new targets for personalized therapies, unique patterns for better diagnosis, and evidence that the oral microbiome is pivotal in understanding MS.