Table 1.

Empirical tests of virulence evolution theory.

key finding	key empirical evidence (selected papers)
virulence and transmission rate are positively correlated through replication rate	Mus musculus/Plasmodium chabaudi [22]; Homo sapiens/Plasmodium falciparum [23]; Daphnia magna/Pasteuria ramosa [24]; Homo sapiens/HIV-1 [25]; Danaus plexippus/Ophryocystis elektroscirrha [26]; meta-analysis of multiple systems [20]
positive trait correlations saturate so that R0 peaks at intermediate virulence	Oryctolagus cuniculus/Myxoma virus [17] (virulence–recovery rate); Homo sapiens/Plasmodium falciparum [23] (virulence–transmission rate); Daphnia magna/Pasteuria ramosa [24] (virulence rate–transmission rate); Homo sapiens/HIV-1 [25] (virulence rate–transmission rate), Danaus plexippus/Ophryocystis elektroscirrha [26] (virulence–transmission rate), Gallus gallus domesticus/Marek's disease virus [27] (virulence–transmission rate), Haemorhous mexicanu/Mycoplasma gallisepticum [28] (virulence–transmission rate)
high susceptible density at the start of an epidemic selects for higher virulence	Escherichia coli/bacteriophage lambda [29]
structured host populations select for less transmissible, prudent strategies	Escherichia coli/T4 coliphage [30]; Plodia interpunctella/granulosis virus [31]; Escherichia coli/bacteriophage lambda [32]
high virulence can trade-off with decreased host movement	Danaus plexippus/Ophryocystis elektroscirrha [33]; Haemorhous mexicanu/Mycoplasma gallisepticum [34]; Paramecium caudatum/Holospora undulata [35]
virulence evolves in natural epidemics of emerging disease	Haemorhous mexicanu/Mycoplasma gallisepticum [36,37] (less virulent strains spread fastest because of movement–virulence trade-offs and then are replaced by higher virulence strains. When hosts start evolving resistance, virulence continues to increase through increased symptom severity rather than through replication rate) Oryctolagus cuniculus/Myxoma virus [38] (lower virulence quickly evolves from extremely high virulence introduction strains. When hosts start evolving resistance, virulence starts to increase) Corvus brachyrhynchos/West Nile Virus [39] (a mutation conferring high virulence in American crows was positively selected, though this may have been a result of selection in another bird or vector species)