Fowl cholera (Pasteurella multocida) is a problem in broiler chicken breeder operations and has emerged in organic and free range broiler production. Existing diagnostic tools along with supporting molecular assays have been used to assess genetic diversity within a fowl cholera outbreak, confirm the identification of the isolate as P. multocida, and then identify the lipopolysaccharide genotype via PCR. However, these tools provide little information about the impact of diversity between isolates on key issues such as immunogenic surface structures and virulence factors. The importance of genetic variation within a pathogen (the concept of a “cloud” as opposed to a single species or indeed a single clone) is well appreciated in poultry vaccines and poultry diseases. This proposal seeks to test this concept in disease outbreaks associated with fowl cholera. A knowledge of the impact of “clouds” will allow a more informed and more targetted use of vaccines and biosecurity programs.