As early as the 13th century, scholars proposed that diseases were caused by ‘invisible entities’ unseen to the naked eye. However, it was not until the 1670s when Antonie van Leeuwenhoek made his microscope did the theory gain traction. Just as seeing tiny ‘animalcules’ zipping around under the microscope enabled the field to advance, it has taken development of the right tools to enable 21st century scientists to uncover the full impact that microorganisms have on human health.