Respiratory diseases constitute a major problem in pig farms, often resulting from multiple infections with different respiratory pathogens. This phenomenon is referred to as porcine respiratory disease complex (PRDC). Swine influenza viruses of type A (swIAV) affect half of French farms and are often isolated from PRDC cases. The flu severity is recognized to be strongly influenced by the presence of other respiratory pathogens but interactions between microorganisms are still poorly understood. Development of experimental models of co-infections is required to better understand mechanisms underlying flu exacerbation that is a prerequisite for improving measures for disease control. This review summarizes current knowledge on flu disease in pig and swIAV's involvement in PRDC, as well as results of in vivo co-infections studies involving swIAVs. The mechanisms responsible for the flu exacerbation in case of co-infection with Mycoplasma hyopneumoniae (Mhp) are more particularly discussed, the swIAV/Mhp experimental model having been the most studied to date. In this case, it appears that the severe flu is due to additive inflammatory responses rather than due to a synergistic effect between both pathogens.