Hypothesis: a selective advantage for cystic fibrosis heterozygotes.

European populations have both a particularly long history of pulmonary tuberculosis and extremely high frequencies of cystic fibrosis (CF). While carriers of the recessive gene are asymptomatic for CF disease, their fibroblasts produce excessive amounts of hyaluronic acid, whose role in successful isolation of virulent pathogens appears to be especially adaptive in host resistance to the human strain of Mycobacterium tuberculosis. CF heterozygosity may have been an adaptation to a disease environment once dominated by this infection.