As the population grew, diseases like typhoid, cholera and malaria thrived. This may explain why so many people carry the sickle-cell and cystic fibrosis (CF) alleles. Both cause serious genetic conditions in people who inherit two copies. But people with just one copy of the sickle-cell allele are unaffected and are resistant to malaria. Some scientists believe that similarly, carriers of CF alleles resist cholera and typhoid.