Parainfluenza Virus 3 The PI3 virus infects the upper respiratory mucosa, where it is shed in aerosols and by direct contact. Unlike IBR (BHV-1) and BVDV, PI3 is responsible only for respiratory disease. In mature cattle, uncomplicated PI3 infection is usually associated with mild or subclinical disease. Fatalities are rare, and usually involve secondary bacterial infection. The most important role of PI3 in bovine respiratory disease (BRD) is to predispose cattle to concurrent infections with IBR virus or bacterial respiratory pathogens.