The common cold is a complex infectious syndrome caused by any one of a large number of antigenitically distinct viruses found in four groups. These groups are the myxo- and paramyxoviruses, the adenoviruses, the rhinoviruses and the coronaviruses. The members of the different groups differ in their physical, biochemical, and immunologic characteristics. With currently available methods, it is possible to determine the cause of 60-70% of colds. The large rhinovirus group is the most important of the known common cold viruses, accounting for approximately 30% of colds. These small RNA viruses have a genome of 7000 nucleotides, which shares considerable homology with poliovirus. The capsid of the rhinovirus is loosely packed, resulting in a relative acid sensitivity compared to the enteroviruses. Although there are at least 89 different antigenic types, all rhinoviruses attach to either one of two cellular receptors. Immunity to rhinovirus is type-specific and associated with neutralizing antibody in nasal secretions and serum. There is a steady acquisition of antibody to the rhinoviruses during childhood and adolescence. The rhinoviruses may be undergoing slow antigenic drift.