Descriptive epidemiological studies

Descriptive epidemiology aims to describe the distribution of disease in terms of one or more of the following:

which animals are affected (animal)
From these studies, an estimate of the proportion of animals infected with a certain pathogen at a specific point time could be made (known as the 'prevalence' of infection), which could be useful in terms of deciding the burden of a disease within the population at that time. Alternatively, an investigation could be conducted in order to estimate the number of new infections with the pathogen over a certain time period (known as the 'incidence' of infection), which could indicate the spread of infection within the population. Of course, studies could be more complex than this, and can utilise a wide variety of statistical methods in order to quantify the level of disease. Studies may also investigate the levels of disease in animals of different ages, sexes, species, etc...

where affected animals are located (place)
Any epidemiological study requires some consideration of the location of animals, as the population of interest must be determined. However, the location of affected animals may be explicitly investigated in some cases. The use of computer programs in order to visualise the spatial patterns of disease has increased in recent years due to improvements in computing capacities, and has become a very useful tool in epidemiology.

when cases occur (time)
Again, although the timing of disease (within the study period) is implicit in any investigation, some studies may investigate temporal patterns of disease further. One example of this is outbreak investigation, where the number of cases of disease within a population over time may be plotted in the form of an 'epidemic curve'. This can provide useful information regarding trends of disease, and may advise upon the effectiveness of any control measures instigated.

Descriptive studies are therefore a very useful tool in the investigation of levels of disease in populations, as well as in the crude monitoring of the effect of an intervention or exposure to a risk factor. However, in a purely descriptive study, no attempt is made to formally investigate reasons for the patterns of disease observed, although hypotheses regarding possible reasons will commonly be generated and developed as a result of these investigations.