1. 94 After the nurse ascertains the diagnosis of disease, she n1akes observations of the disease frequency and recorded as disease rates. The attack rate (Valanis, 1992) is used to calculate an identifiable populatio n exposed to an infectious agent. It represents the incidence of the illness among the exposed population. They are frequently used in surveillance and control of communicable diseases. 2. Describing disease as to person, place and time characteristics After the disease or condition has been identified with reasonable certainty, the number of persons who possess the disease are recorded noting down the characteristics of the afflicted persons, the time the disease was initially recognized and the characteristics of the place where the cases came from. To a certain extent, the nurse can derive associations between the risk of acquiring a disease and characteristics of persons. Some variables provide clues as to the probable cause of the disease. These variables include age, sex, marital conditions, occupation or socio-economic status. In viewing the susceptibility of the community as host, the nurse determ ines the characteristics of the community and its population in terms of the following: 1. Herd immunity is tbe basis for determining the community's reaction against disease invasion since it represents the immunity and susceptib ility levels of individuals comprising the populatio n. Tbe immunity level is inversely proportional to the soscepbbility level. When the proportion ofthesusceptib le are high compared to the proportion of tbe immunes, then the community is experie ncing an epidemic. An epidemic is a situation when there is a marked upward fluctuation in disease incidence. An cnd. emic occurrence of disease implies the habitual presence of diseas,,> in a given geographic location accounting for the low number of both immunes and susceptib le. Wb. en disease occurs every now and then affecting only a small number of people relative to the total population, then we have a sporadic type of disease oceurrence. 2. Exposure or Contact Rate represents opportunities for progressive opportunities for progressive transfer or transmission of an infectious agent to a susceptib le host and depends on the frequency of contact. and facility of transmission. 3. Chance is the probability of contact between the source of infection and the susceptib le host and depends upon the number of sources of infection, the number of immunes and location of the source of infection. 