Cystic fibrosis is a recessive monogenetic condition which is devastating for the evolutionary fitness of the sufferer. The disorder has not been eradicated by human evolution and allele frequencies as high as (1:20) are seen in Caucasian populations. The potential explanations for the persistence of the disease are discussed including a high rate of genetic mutation, the founder effect and heterozygote advantage. Three firm conclusions are reached; the first is that the most influential factor in causing the high CF prevalence amongst peoples in European ancestry is heterozygote advantage to Tuberculosis. Secondly, the founder effect accounts for the additional amplification of allele frequency in emigrant communities. Finally, new mutations help to subsidies the mutation pool at a linear rate across all ethnicities.