Complex sentence: Individuals in a population have frequently different haplotypes and as a result many combinations, even in small groups, affords the survival of the groups and thwarts evolution of epitopes in pathogens to hide from the immune system.
Split into simpler sentences: Individuals in a population frequently have different haplotypes, and this results in many combinations, even in small groups. This diversity enhances the survival of such groups, and thwarts evolution of epitopes in pathogens, which would otherwise be able to be shielded from the immune system.