India, the Mediterranean and the Middle East have higher allele frequencies, as well. As effective antimalarial treatment becomes increasingly available to malaria-stricken populations, the allele frequency for SCA is expected to decrease, so long as SCA treatments are unavailable or only partially effective. If effective sickle-cell anemia treatments become available to the same degree, allele frequencies should remain at their present levels in these populations. In this context, 'treatment effectiveness' refers to the reproductive fitness it grants, rather than the degree of suffering alleviation. Cystic fibrosis Cystic fibrosis (CF) is an autosomal recessive hereditary monogenic disease of the lungs, sweat glands and digestive system. The disorder is caused by the malfunction of the CFTR protein, which controls intermembrane transport of chloride ions, which is vital to maintaining equilibrium of water in the body. The malfunctioning protein causes viscous mucus to form in the lungs and intestinal tract. Before modern times, children born with CF would have a life expectancy of only a few years, but modern medicine has made it possible for these people to live into adulthood. However, even in these individuals, CF typically causes male infertility. It is the most common genetic disease among people of European descent. The presence of a single CF mutation may influence survival of people affected by diseases involving loss of body fluid, typically due to diarrhea. The most common of these maladies is cholera, which only began killing Europeans millennia after the CF mutation frequency was already established in the population. Another such disease that CF may protect against is typhoid. Those with cholera would often die of dehydration due to intestinal water losses. A mouse model of CF was used to study resistance to cholera, and the results were published in Science in 1994 (Gabriel, et al.). The heterozygote (carrier) mouse had less secretory diarrhea than normal, noncarrier mice. Thus, it appeared for a time that resistance to cholera explained the selective advantage to being a carrier for CF and why the carrier state was so frequent. This theory has been called into question. Hogenauer, et al. have challenged this popular theory with a human study. Prior data were based solely on mouse experiments. These authors found the heterozygote state was indistinguishable from the noncarrier state. Another theory for the prevalence of the CF mutation is that it provides resistance to tuberculosis. Tuberculosis was responsible for 20% of all European deaths