The causes of BRD are multiple and complex, but the three factors of stress, viral infection and bacterial infection are almost always involved in cases of severe disease. A wide variety of different stressors and agents may be involved in the disease process.
Some of the viral agents produce only mild clinical signs by themselves but when combined with other viral or bacterial agents and stress they may cause severe signs and death. Many normal cattle carry one or more of the bacterial and viral agents in their upper respiratory system with no ill effects. These often enter the lungs but are usually expelled or inactivated. However, under stress the animal’s defense mechanisms may be overcome, and the infection established, resulting in BRD. The mixing of cattle from different sources and wide environmental temperature fluctuations have been identified as major factors in the initiation of disease outbreaks in feedlots.
Source: Clell V. Bagley, DVM, Extension Veterinarian, UtahStateUniversity