Heterogeneity in pathogen transmission: mechanisms and methodology

Heterogeneity in the ability of hosts to transmit pathogens is among the most fundamental concepts in disease dynamics and has major implications for disease control strategies. The number of secondary infections produced by an infected individual is a function of three components: an individual's infectiousness, the rate at which it contacts susceptible individuals and the duration of infection. Individual-level variation can emerge in each of these components through a combination of behavioural and physiological mechanisms. In this review, we describe mechanisms that promote variation in the number of individuals to which an individual transmits a pathogen, emphasizing insights that can be gained by understanding which components of transmission (infectiousness, contact rate, infection duration) are primarily affected. We also discuss how behavioural and physiological processes generate transmission heterogeneities across multiple scales, from individual-level variation to heterogeneity among species. Strategies for quantifying each transmission component are presented, and we discuss why studies focusing on only one component of the infection process may be misleading. To conclude, we describe how future research focusing on variation in transmission across all three components can provide a more holistic view of heterogeneity in pathogen transmission.