Epidemics by Transmission An epidemic is an extraordinary event, namely the emergence of a disease that affects people in an area that exceeds normal estimates of occurrence in a short period. 1. Epidemics According to Transmission Method a) Common Source Epidemic This epidemic is often called a "point epidemic", namely an epidemic that occurs suddenly in a short time as a result of joint exposure to a disease-causing agent. Common source epidemics usually cover a period of time and spread is limited to certain people, times and places, for example food poisoning or drinks contaminated by pathogenic microorganisms. b) Person-to-Person Epidemics This epidemic means an extraordinary event where transmission occurs from infected people to other susceptible people. This epidemic is often called a progressive epidemic. This epidemic form occurs due to person-to-person transmission, either directly or indirectly. The occurrence of this kind of epidemic is relatively longer in time according to the nature of the disease and the length of the germination period. It is also greatly influenced by population density and the distribution of community members who are susceptible to the disease. The period of emergence of the disease mentioned above is around one month so it appears that the epidemic period is quite long with the situation increasing in the number of sufferers from time to time until the time when the number of susceptible community members reaches a minimum limit. Several things you need to know and pay attention to in a progressive epidemic are: 1) Generation Time 2) Herd Immunity 3) Attack Rate and Average Attack Rate Generation Time After pathogenic microorganisms enter the human body, they multiply and do not cause symptoms. This person is a potential source of infection. The ability to transmit disease is not strong at first, but gets stronger over time until it reaches a peak and then decreases again.The time interval for the spread of disease from person to person is determined by generation time, namely the period between being infected by someone and when transmission is maximum. Generation time is similar to budding time, but with the following differences: a. Generation time can occur before or after the period shoots b. Generation time can occur in infectious diseases that cause symptoms or do not cause symptoms. This is a difference from the budding period which only occurs in infectious diseases that cause symptoms. Thus it can be concluded that the generation time is more dangerous than the budding period, especially if the generation time occurs before the budding period because people do not know that this condition is very contagious. Herd Immunity Herd immunity is immunity possessed by some of the population which can prevent the spread. This can be equated with an individual's level of immunity, that is, the higher a person's level of immunity, the more difficult it is to catch the disease. Likewise with herd immunity, the greater the proportion of the population who is immune means the higher the level of herd immunity , the more difficult it is to spread the disease. The existence of herd immunity is clearly visible in epidemics progressive because when an epidemic occurs, not everyone is susceptible to disease and the incidence of epidemics occurs in progressive epidemics. The prevalence of an epidemic occurs when in an area there are many people who are susceptible to a particular disease and have herd immunity. The population's exposure to the disease in question is low, which means that residents of the area are easily affected by outbreaks. After an outbreak occurs, the number of people who are immune increases until herd immunity increases until the spread of the disease stops. After some time, the number of people who are immune decreases as does herd immunity and the disease outbreak comes again, and so on. Progressive epidemics easily occur in virgin populations and closed populations. Virgin population is a population that has never been exposed to a particular agent, or has been exposed but has long since disappeared. This condition results in many residents being susceptible to the disease or having low herd immunity . Likewise with closed communities , namely isolated communities. Attack Rate and Average Attack Rate Attack rate is the number of incidents that occur in a short time. Another important aspect in the process of disease transmission is the procedures and concepts of family life, the system of family relationships in society and the system of relationships between individuals in daily life in certain population groups which are epidemiological units where the spread of disease can take place. Cases or sufferers of certain infectious diseases that arise in a particular family or group of residents who become a point of attention for public health officials are called index cases. The spread of disease in a particular group can be measured by the attack rate called the Scondary Attack Rate , namely the number of cases that develop/appear in a certain time unit among group members who experience contact and have a certain risk or vulnerability to the disease.