M haemolytica and B trehalosi are distributed worldwide, and diseases caused by them are common in sheep and goats of all ages, although the prevalence of serotypes may vary by region and flock. M haemolytica, B trehalosi, and P multocida are common commensal organisms of the tonsils and nasopharynx of healthy sheep and goats. The presence of multiple Pasteurella spp may serve to keep the bacterial populations in check, because there appears to be some interference with growth when multiple species are present. For these organisms to cause infection, a combination of stressors, including heat, overcrowding, exposure to inclement weather, poor ventilation, handling, and transportation, leaves sheep and goats susceptible to respiratory viral infections. Parainfluenza 3, adenovirus type 6, respiratory syncytial virus, possibly bovine adenovirus type 2, ovine adenovirus types 1 and 5, and reovirus type 1 cause primary respiratory infections that are rarely life threatening but predispose to secondary M haemolytica infections. Respiratory infections with Mycoplasma ovipneumoniae and Bordetella parapertussis have also been reported to be associated with secondary M haemolytica infections. The combination of stressors and primary infections are thought to break down the mucosal barrier integrity of the lower respiratory tract, allowing M haemolytica to colonize, proliferate, and induce significant tissue damage.