Effect of e (mean contribution of each individual to water contamination) on the threshold population size S
C (equation 5). Curve a = 0.5 uses parameters from community 1; curve a = 1 uses parameters from communities 2 and 3 (table 2). Each curve defines values of e and S
C at which R0 = 1 (see additional material: Appendix). A community located above its curve will suffer an outbreak following the introduction of infectives. A community located below its curve, will not. The three hypothetical communities occupy the same position in this graph as they share the same initial susceptible pool size and degree of water contamination. However, community 1 does not suffer a cholera outbreak because the lower contact with contaminated water (a = 0.5) shifts its threshold curve outwards. Communities 2 and 3 are located above their threshold curve (a = 1); the introduction of infectives will trigger an outbreak.