Why can a mutation that harms health still spread through a population?

In the context of Darwinian medicine, a mutation that harms health can persist or spread through a population due to several evolutionary mechanisms:

• Reproductive Advantage: Natural selection shapes bodies for maximum reproduction rather than optimal health. Mutations that increase reproductive success tend to spread, even if they decrease longevity or overall health.1
• Heterozygote Advantage: Sometimes, carrying one copy of a deleterious allele provides a survival benefit in specific environments. For example, the gene for sickle cell anemia persists in malarial regions because individuals who are heterozygous for the trait have increased resistance to malaria.2
• Sheltering in Heterozygotes: Recessive deleterious alleles are often "sheltered" in heterozygotes, who do not express the harmful phenotype. Because these individuals remain healthy and reproduce, the allele is not efficiently removed by natural selection and can remain in the population.2
• Mutation-Selection Equilibrium: Deleterious alleles are never completely eliminated because new mutations continuously arise. A dynamic equilibrium is reached where the rate of new mutations balances the rate at which selection removes them.3