Genetic elements that spread by parasitizing the replicative machinery of a host cell are common inhabitants of the genome, especially in multicellular sexual organisms. They show how genetic elements can evolve despite reducing the fitness of individuals. The first section in this chapter is all about infection and explains the following: bacterial plasmids; 2-micron plasmid of yeast; transposable elements; transposons as regulatory elements; transposons as mutator elements; transposons as hitch-hiking elements; transposons as selfish genetic elements; the spread of transposable elements; parasites of transposons'; selfish mitochondria; and finally population genetic engineering. The second section is about interference and describes the Poison-Antidote system; Gamete Killers; meiotic drive on sex-chromosomes; and phoenix elements. The final section in this chapter, called Gonotaxis, describes B-chromosome; and infectious agents that control sexual development.