Sexual reproduction can be maintained only in an ever-changing world of diseases generating a never-ending coevolutionary arms race between infectious diseases and their hosts. The polymorphic Major Histocompatibility Complex (MHC) allows vertebrates including humans to track changing parasites through olfactory mate choice for the partner that offers the currently optimal complementary set of MHC alleles. The extremely high standing variation with more than 1000 different MHC alleles present also in human populations offers ample opportunity for choice. What is needed to respond to a new parasite mostly already exists and does not need to be invented through mutations. Adapting to locally different parasites can induce speciation through locally adapted sets of immunogenes.