Evolution is simple if adaptive mutations appear one at a time. However, in large microbial populations many mutations arise simultaneously resulting in a complex dynamics of competing variants. I will discuss recent insight into universal properties of such rapidly adapting populations and compare model predictions to whole genome deep sequencing data of HIV-1 populations at many consecutive time points. Genetic diversity data can further be used to infer fitness of individuals in a population sample and predict successful genotypes. We validate these prediction using historical influenza virus sequence data. Successful predictions of the composition of future influenza virus population could guide strain selection for seasonal influenza vaccines.