Editing Parasitism (section)

==== Competition favouring virulence ====

Competition between parasites can be expected to favour faster reproducing and therefore more [[Virulence|virulent]] parasites, by [[natural selection]].<ref name=MasseyBuckling2004>{{cite journal |last1=Massey |first1=R. C. |last2=Buckling |first2=A. |last3=ffrench-Constant |first3=R.|author1-link=Ruth Massey |title=Interference competition and parasite virulence |journal=Proceedings of the Royal Society B: Biological Sciences |volume=271 |issue=1541 |year=2004 |pages=785–788 |doi=10.1098/rspb.2004.2676 |pmc=1691666 |pmid=15255095}}</ref><ref name=Rigaud2010>{{cite journal |last1=Rigaud |first1=T. |last2=Perrot-Minnot |first2=M.-J. |last3=Brown |first3=M. J. F. |title=Parasite and host assemblages: embracing the reality will improve our knowledge of parasite transmission and virulence |journal=Proceedings of the Royal Society B: Biological Sciences |volume=277 |issue=1701 |year=2010 |pages=3693–3702 |doi=10.1098/rspb.2010.1163|pmid=20667874 |pmc=2992712 }}</ref>

[[File:Pink Flamingos with Duck - Camargue, France - April 2007 (cropped).jpg|thumb|Biologists long suspected [[cospeciation]] of [[flamingo]]s and [[duck]]s with their parasitic [[louse|lice]], which were similar in the two families. Cospeciation did occur, but it led to flamingos and [[grebe]]s, with a later [[host switch]] of flamingo lice to ducks.]]

Among competing parasitic insect-killing bacteria of the genera ''[[Photorhabdus]]'' and ''[[Xenorhabdus]]'', virulence depended on the relative potency of the antimicrobial [[toxin]]s ([[bacteriocins]]) produced by the two strains involved. When only one bacterium could kill the other, the other strain was excluded by the competition. But when [[caterpillar]]s were infected with bacteria both of which had toxins able to kill the other strain, neither strain was excluded, and their virulence was less than when the insect was infected by a single strain.<ref name=MasseyBuckling2004/>