In bacteria, plasmids can move horizontally between cells of the same and different species through the process of conjugation. When a plasmid imposes a fitness cost on its bacterial host, a sufficiently high level of conjugation is required to maintain the extrachromosomal element in the population (effectively as a molecular parasite). For costly plasmids with low conjugation rates, their long-term persistence presents a paradox. Prime examples of this paradoxical persistence concern plasmids that house antibiotic resistance genes, which can be costly in the absence of antibiotics.