Our intestines harbor an astronomical number of bacteria, around 100 times the number of cells in our body, known as the gut microbiota. These bacteria belong to thousands of species that co-exist, interact with each other and are key to our health. While it is clear that species imbalances may result in disease, it is unclear at what pace does each species in the gut evolves, a process that contributes to the chance of a particular innocuous species becoming harmful to the host.