Development of successful vaccines against human infectious diseases depends on using appropriate animal models for testing vaccine efficacy and safety. For some viral infections the task is further complicated by the frequently changing genetic make-up of the virus, as in the case of influenza, or by the existence of the little-understood phenomenon of vaccine-enhanced disease, as in the case of respiratory syncytial virus (RSV). The cotton rat Sigmodon hispidus has been used for years as an excellent small animal model of the RSV vaccine-enhanced disease. Recently, using cotton rats, we have demonstrated that vaccination against another paramyxovirus, human metapneumovirus (hMPV), can also lead to vaccine-enhanced disease. In addition to the study of paramyxoviruses, S. hispidus presents important advantages for the study of orthomyxoviruses such as influenza. The cotton rat is susceptible to infection with unadapted human influenza strains, and heterosubtypic immunity to influenza can be evoked in S. hispidus. The mechanisms of influenza, RSV, and hMPV pathogenesis and immunity can now be investigated in the cotton rat with the development of species-specific reagents for this animal model.