Epidemics are the occurrence of a number of cases of disease highly in excess of that expected in a given place and time. Some epidemics have features of both common-source epidemics and propagated epidemics. Some epidemics are classified as other in that they are neither ‘common source’ in its usual sense nor propagated from person to person. Outbreaks of zoonotic or vector-borne disease can result from sufficient prevalence of infection in host species, sufficient presence of vectors and sufficient human–vector interaction. Although the epidemic threshold is calculated by month, the data manager can use the cumulative number of cases reported for the month as a guide against the epidemic threshold. The epidemic threshold for the month of January was determined to be 120 cases for a particular catchment area of a health facility/sentinel site. The expected number of cases for each week of January would be approximately 30 cases per week.