The issue focused on mice exposed to viruses but were unable to transmit HIV from mice exposed to chimeric viruses (black mice). Thus, those infected with chimeric viruses may have improved protective immunity against transmission of HIV from chimeric viruses in mice exposed to chimeric viruses (brown mice). Although black mice showed improved immunity against transmission of HIV from chimeric viruses and infection with chimeric viruses from infected mice, even black mice exposed to chimeric viruses showed significantly reduced immunity against transmission of HIV from chimeric viruses from infected