Pasteurella multocida, the cause of avian cholera, has naturally infected over 100 species of free-living birds. Among wild birds, avian cholera has its greatest impact on North American wildfowl. Epizootics usually are explosive in onset and may involve thousands of birds. The disease has been reported in every month of the year among wildfowl. Disproportionate mortality, with some species suffering proportionately greater mortality than others, has been a common feature of this disease. Presence of animal organic matter plays a significant role in the survival of P. multocida. There are conflicting reports or a lack of information on the role of host sex, age, body size, other physical features, genetic variation or behavioral differences, as predisposing factors to infection by P. multocida. There also are ambiguities on the relationship between season, precipitation, temperature, nutritional stress, water quality, other microorganisms, and environmental contaminants, and the occurrence of avian cholera in wildfowl. Two competing hypotheses for the year-round reservoir of wildfowl strains of P. multocida are ambient soil or water of enzootic sites, and carrier animals; most current evidence favors the role of carrier animals. Transmission most likely occurs by ingestion of contaminated water, inhalation of bacteriarich aerosols, or both. While many techniques have been proposed to prevent or control avian cholera, none have been rigorously tested to determine their effectiveness.