The AIDS epidemic has existed for over 20 years. During this time, HIV-1 has demonstrated an extraordinary ability to mutate and evolve. The capacity of HIV-1 to change is demonstrated by comparison with the variability of another viral pathogen, influenza (Fig. 4 The magnitude of HIV-1's evolutionary potential is illustrated by comparing it with that of another rapidly evolving virus, influenza. The scale bar, which equals 10% variation, is the same in each panel. Phylogenetic trees based on the indicated sequences are shown. (a) Twenty HA1 domain sequences of A/Sydney-like viruses circulating in Canada during the first half of the 1997 to 1998 flu season. (b) All 96 HA1 domain sequences of human influenza H3N2 viruses in the Influenza Sequence Database, Los Alamos National Laboratory (http://www.flu.lanl.gov) with an isolation year of 1996. (c) Nine env V2-C5 sequences from a single asymptomatic individual collected at one time point 73 months postseroconversion this was a subtype B infection, and is typical of intrapatient diversity . (d) HIV-1 subtype CRF03_AB V2-C5 sequences from 26 individuals from Kaliningrad. This outbreak represents a unique situation where a recombinant form of the virus spread explosively through a population of intravenous drug users, and all viruses were very closely related to a single common ancestor . These samples were collected during 1997 to 1998, within a year of the introduction of the strain into the population. (e) HIV-1 V2-C5 env sequences from a subtype B epidemic, sampled from 23 individuals residing in Amsterdam in 1990 to 1991 . (f) HIV-1 V2-C5 sequences sampled in 1997 from 193 individuals residing in the Democratic Republic of the Congo. Note the difference in degree of diversity for influenza compared with that of HIV-1 . (From Korber B, Gaschen B, Yusim K, Thakallapally R, Kesmir C, Detours V. Evolutionary and immunological implications of contemporary HIV-1 variation. Br Med Bull 2001;58:19-42, with permission.)). This plasticity presents many challenges to treatment and prevention. An increased understanding of the ability of HIV-1 to evolve should assist in the development of better therapeutic and vaccine strategies.
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