An age-structured model is used to study the possible impact of drug treatment of infections with the human immunodeficiency virus type 1 (HIV-1) on evolution of the pathogen. Inappropriate drug therapy offen leads to the development of drug-resistant mutants of the virus. Previous studies have shown that natural selection within a host favors viruses that maximize their fitness. By demonstrating how drug therapy may influence the within host viral fitness
we show that while a higher treatment efficacy reduces the fitness of the drug-sensitive virus, it may provide a stronger force of selection for drug-resistant viruses allowing a wider range of resistant strains to invade.
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