**4.1 General outcome of disease**

Three possible clinical outcomes have been shown to occur with AI infection in birds: there may be no clinical signs, it may result in a mild disease, and severe disease with death may occur [3]. Virus replication within the cell, tissue, organ, or a combination results in pathobiological changes which are abnormal physiological and anatomic changes. Therefore, in general, as virus replication titers increase, so do the severity of pathobiological changes such as gross and microscopic lesions with the most pathogenic virus strains causing major cell damage and death if it is sufficiently severe to affect critical organs [3] **Figure 4**.
