2.2.4 Time: when do health problems occur?
The question of when (in time) diseases occur or peak is of considerable interest in epidemiology. For example, it is well established that a range of well-known infectious diseases (e.g. measles, influenza and whooping cough) show cyclical variations in occurrence, which result in epidemics every few years.
Thinking point: Look at Figure 1 and identify in which years notifications about pertussis (whooping cough) cases peaked.
You will note that, in 1978, over 60,000 cases were reported and this fell dramatically in 1981, only to rise again in 1982. The rises may be explained by media coverage of the dangers of the vaccine in the mid-1970s and then again in the early 1980s.
When considering time in relation to the occurrence of diseases, it has to be noted that time can be measured in a variety of ways: as secular time (referring to centuries), as cyclical time or time intervals such as five yearly, as seasonal time (i.e. summer, autumn, winter and spring), or by specifying particular times of the week or times of the day. In relation to ‘secular time’ in the UK and elsewhere, infectious diseases constituted a major health problem throughout the nineteenth century. Fortunately, most of these diseases have declined and some, such as smallpox, have almost been eliminated. By the end of the twentieth century, however, heart disease and cancer had become the major health problems.
Seasonal variations in the incidence of disease are most common for respiratory tract infections (during winter months). Salmonella food poisoning also frequently shows seasonal variations, with peaks during the summer and in the Christmas period. Hay fever and other allergies occur primarily in the early summer.
Outbreaks of diseases can also be related to specific points in time, locations or events. For example, the sudden occurrence of infection such as typhoid or salmonella food poisoning is usually due to the simultaneous (or near simultaneous) exposure of groups of susceptible people to a certain micro-organism, as might happen at a wedding reception or in a hotel.
Despite considerable evidence about the influence various factors have on people’s health, epidemiologists cannot infer that the link between a factor and illhealth is a causal one. However, descriptive studies give an indication of the cause of a disease, but special studies in epidemiology can explore causation further.