Condition | Significance | Evidence |
---|---|---|
Trade-off between rate of transmissibility (infectiousness) and host life span | Required to make “optimal virulence” dependent on the background rate of host mortality | Symptoms (morbidity) can aid transmission: diarrhea, coughs, bacteriuria |
Detection of host health (mortality) | Required to sense changes in the selection regime (due to changing host mortality) | Bacterial sensing of host stress molecules turns on virulence phenotype [27] |
Alternating selection regimes (mortality pattern) in the host population | Required to impose selection pressure from both regimes (favoring low and high virulence) | Extended aging in humans |
Variability of microbiome composition among connected host individuals | Required to make between-host spread (strongly) dependent on instantaneous transmissibility | Large compositional variability in the human population [11], even within families [32] |