An epidemic disease is a disease that occurs in a population in excess of its normally expected frequency of occurrence. In an epidemic disease, disease events are clustered in time and space. Note that a disease may be epidemic even at a low frequency of occurrence, provided that it occurs in excess of its expected frequency. A pandemic is a large epidemic affecting several countries or even one or more continents. A sporadic disease is a disease that is normally absent from a population but which can occur in that population, although rarely and without predictable regularity. Many epidemics of infectious disease occur in a regular, cyclical fashion over a prolonged period of time. This is because with an increasing frequency of occurrence of the disease in a host population, the number of susceptible hosts decreases as individuals within that population become infected, and then either die or recover and become immune to reinfection. As the number of susceptible hosts decreases, so does the opportunity for disease transmission. This, in turn, means that the frequency of occurrence of new cases of the disease declines. A period of time then elapses during which new susceptible individuals are born into the host population. The number of susceptible hosts in the population thus increases, and the opportunities for the disease agent to find a susceptible host are enhanced. As a result the frequency of occurrence of the disease may increase and a new epidemic may take place. When assessing the efficacy of measures introduced to control epidemics, an attempt should be made to distinguish between a decline in the frequency of occurrence of the disease due to a control measure, and a natural decline in the epidemic cycle. Epidemics can be prevented if the level of immunity in the host population can be sustained. It is important, therefore, in instances where the control of an infectious disease is being attempted by vaccination, that coverage be maintained in the. host population even when the disease is occurring rarely. Quantification of disease events. Any description of a disease problem should include an attempt at quantification. The methods by which disease events in populations are quantified are described in Chapter 3.
Certain time trends can be related to the type of death. In the United Kingdom , for example, the steady rise in suicides from 1945 to 1962-63 was probably to some extent curtailed following the removal of carbon monoxide from domestic gas supplies which occurred with the change from coal gas to natural gas during the sixties.  Methods vary across cultures, and the easy availability of lethal agents and materials plays a role. A 2014 overview of China's suicide rates noted they have been dropping by 63% in 20 years, through China 's late urbanisation and migration from rural areas, where poisonous pesticides are used to suicide. The same study (among others) cites financial crises as periods of greater suicide rates.