Recovery and identification of human myxoviruses.

The myxoviruses which infect man can be divided into two major subgroups (Table 1): the influenza subgroup which includes influenza virus types A, B, and C and the noninfluenza subgroup which includes parainfluenza types 1 (HA-2), 2 (CA), 3 (HA-1), and 4, mumps, measles, and respiratory syncytial viruses. Related but distinct antigenic variants or subtypes have been detected among types of both subgroups. In this manner, three subtypes are included among influenza virus type A: A, Al, and A2 (16, 17, 18). Influenza virus type B comprises three subtypes: B, B1, and B2 (16, 17, 23). Recently, two subtypes, A and B, were recognized for parainfluenza type 4 virus (4). In addition, at least two different antigenic variants have been detected among respiratory syncytial virus strains (7).