Evolutionary baggage is the part of the genome of a population that was advantageous in past individuals but is disadvantageous under the pressures exerted by natural selection today.
Natural selection is not a perfect process; if an organism is “fit enough” to survive a particular environment and reproduce, its genes are passed on to the next generation.
This seeming paradox is the origin of evolutionary baggage, which is the collectively inherited traits that evolved in a different environment from the present.
As detrimental the effects of sickle-cell disease seem, it also offers an unforeseen benefit; humans with the sickle-cell gene show less severe symptoms when infected with malaria, as the abnormal shape of blood cells caused by the disease hinder the malaria parasite's ability to invade and replicate within these cells.
Malaria is caused by parasitic protozoans transferred through mosquito saliva into a person's circulatory system, where they travel to the liver to mature.