A mutation in DNA affects the entire protiens synthesis process by producing the incorrect amino acids in the wrong places, causing issues with the ability of the organism to survive depending on what the protein is in charge of.
During an influenza outbreak, many people get infected and die. However, part of the population acquires immunity and survives. This is due to variations in the genotype and phenotype among individuals in the population. Probably a particular phenotype confers resistance to influenza. Therefore, this population is able to grow and develop to maturity and reproduce. Their genes are therefore passed over to subsequent generations.
