SIVsmm infection in SM, which have an intermediate prevalence of CCR5-null individuals (8% in this population), demonstrate an intermediate relationship, in which alternative coreceptors efficiently mediate both natural and experimental infection in the absence of CCR5, but CCR5 use by the virus is retained, perhaps because both CCR5 and alternative pathways together maximize replication in the majority of animals, while the non-CCR5 pathways are required in the CCR5-null animals. The gene discussed is CCR5; the disease is infection.