A genetically caused disease that generally results in the death of the person with it unless medical interventions are available. Sickle cell anemia is a popular topic for biology courses because it is one the few, well-worked out examples of heterozygote advantage that we have. People carrying two copies of the sickle cell allele have the disease, people with no copies of the sickle cell allele are normal, but people carrying just one copy of the sickle cell allele are resistant to malaria (though they may occasionally have symptoms of sickle cell). So, if you live in a region where malaria is common, you are at an advantage if you are a heterozygote (i.e., if you carry one sickle cell allele and one normal allele). For a more detailed explanation, see our resource on sickle cell anemia in Evolution 101.