Autoimmune disorders are still one of the big medical mysteries, though scientists are picking away at the interaction of genes and the environment they believe to be responsible.
Alzheimer's is probably caused by a complex interaction of many genes, and it is possible those with the APOE4 gene are less able to repair their injured brains.
How microbes are modified by individual lifestyle, environmental exposure and interaction with human genes and disease is part of an ambitious research agenda scientists hope will lead to new strategies, medicines and foods to maintain health and treat disease.