8. Epidemiology of infectious diseases
Epidemiology is ‘the study of the occurrence, distribution and control of disease in populations’. The risk of infection depends upon more than simply an individual’s susceptibility; it is also affected by the level of disease within the population, the degree of population mixing and ‘herd immunity’, as well as specific features such as the communicable period, route and ease (infectiousness) of transmission.
Route of transmission
For an infectious agent to persist within a population there must be a cycle of transmission from a contaminated source, through a portal of entry, into a susceptible host and on again (Fig. 3.8.1).
Direct transmission. This is the most common and important route. It involves all forms of physical contact between humans, including sexual transmission, faecal–oral spread and direct respiratory spread via large droplets.