At risk for infections with Campylobacter fetus are elderly and immunocompromised persons and those with occupational exposure to infected animals. Considered reservoirs of C. fetus are cattle and sheep in which this organism is associated with abortion and infertility. C. fetus comprises three subspecies that differ in their primary habitats, and can be divided in different genomics clades, but if there are virulence differences is still unknown. We apply comparative genomics in combination with functional studies to study the epidemiology of C. fetus subspecies and their virulence. As we host the WHO Collaborating Centre for Campylobacter /OIE Reference Laboratory for Campylobacteriosis, we use these methods also for validation of C. fetus diagnostic tests.