Populations at risk: these can be identified by studying the distribution of the disease within host populations by species, breed, age and sex. Descriptions of population densities and movements are also of great value, particularly when the disease is transmitted by contact. Distribution of disease: events in time and space. clustering of disease events in time, space or both. The clustering of disease: events in time may indicate that the host population was exposed to a common source of the disease or its determinant. Seasonal clustering of disease events often indicates the influence of climatic determinants in some form or other. The distribution of disease: events in populations in time and space can be described by three basic descriptive terms. An endemic disease: is a disease that occurs in a population with predictable regularity and with only minor deviations from its expected frequency of occurrence. In endemic diseases, disease events are clustered in space but not in time.