An interesting observation about HIV-1 type C, the predominant form of the virus in sub-Saharan Africa, is that it rarely makes the switch from using CCR5 as the co-receptor with CD4 antigen in the initial infection to CXCR5 as the immune system becomes depressed. It has been suggested that since the CCR5-interacting form of the virus persists in Africa, people infected with type C will have more of the virus capable of initiating an infection, thus enhancing the spread of type C virus. Type C virus may thus spread more effectively in heterosexual sex, the major mode of transmission in Africa.