Principles of disease transmission
The main determinants that influence disease transmission can be classified as the host, the environment and the agent – commonly referred to as the epidemiological triad. The agent is the organism that causes the infection, the host is the potentially susceptible individual and the environment refers to the external factors that affect potential disease transmission. Figure 28a shows some of the key factors impacting on each of the three determinants.
The transmission of a particular infection depends on a number of factors as outlined earlier. Key factors include the infection itself and the underlying population immunity. This is governed by the reproductive number (see Figure 28b). The basic reproductive number is the number of new cases that occur in a totally susceptible population. The effective or net reproductive number is the number of new cases that occur in a population where there may be both susceptible and immune people present. Mathematically, the net reproductive number is the product of the basic reproductive number and the proportion of susceptible individuals. For any infection to stop transmission, the effective reproductive number has to fall below 1: that is, each case infects less than 1 person. This forms the principle of herd immunity (see Chapter 31). Although infections will tend to die out if the reproductive number is less than 1 on average, this assumes homogenous mixing patterns, where all instances of contact in the population are equally likely and, therefore, there is an equal chance for anyone to be potentially infected. In reality, mixing patterns are far from homogenous and transmission can, therefore, occur in groups where the reproductive number is greater than 1, even though the population average may be less than 1.