There is huge variation across pathogen species in the number of cells required to successfully infect a host. This number is known as the ‘infective dose’. At one end of the scale, species such as Shigella and Giardia lamblia require about 10 cells to start an infection. In contrast, species such as Vibrio cholera and Staphylococcus aureus require 103–108 cells in order for an infection to develop. It is unclear why infective dose varies, with large differences occurring even between closely related pathogens.
Mechanisms of Pathogenesis, Infective Dose and Virulence in Human Parasites. (2012) PLoS Pathog 8(2): e1002512. doi:10.1371/journal.ppat.1002512
We found that mechanisms used by parasites to infect hosts are able to explain variation in two key pathogen traits: infective dose and virulence. In pathogens where the molecules secreted to facilitate infection acted locally, the number of cells required to start an infection (infective dose), was lower than in pathogens where the secreted molecules act more distantly. Parasite virulence showed no correlation with local versus distant action, but was negatively correlated with infective dose, and greater in species that infect via wounded skin. By showing how such parasite life history details matter, our results help explain why classical trade-off models have been relatively unsuccessful in explaining broad scale variation across parasite species.