Epidemiology is that the study of the distribution and determinants of disease and therefore the application of this data to the prevention and control of health problems. Epidemiologists view disease mainly at the public level, reporting its incidence and regularity, secular trends, geographic collection, and other patterns. An epidemiological investigation may cause inconclusive results if all of the danger factors or sources of exposure aren't considered or if random or systematic error has occurred. Investigations may fail to watch an association with water or underestimate the danger due to low statistical power or non-differential exposure classification (e.g., obtaining incomplete information about water contact and consumption). Systematic error, like recall bias, also can cause misleading results. Waterborne diseases are usually caused by exposure to enteric pathogens that are transmitted by the faecal–oral route and infrequently by exposure to pathogens in urine (e.g., Leptospira). The pathogens are excreted by infected animals or persons, who may or might not exhibit symptoms. Transmission of those pathogens can occur within the sort of contaminated water, food, or fomites and get in touch with with infected persons or animals. Waterborne infections that don't end in clinically recognized disease are going to be difficult to spot and should not be considered within the risk estimate. However, asymptomatic persons are often a source of contamination and infection. Studies may consider only the first mode of transmission (e.g., water), but secondary transmission can occur. Persons who are infected by contaminated water may infect others. Transmission are often direct or indirect. The transferal of waterborne diseases to domestic, institutional, or other contacts by a prime case has been confirmed epidemiologically in outburst caused by E. coli O157:H7 and Cryptosporidium. The connection between the host, agent, and environment is described by the epidemiological triad, a comparatively simple, but important, model of disease transmission. The host, agent, and environment co-exist independently, and infection occurs only there's interaction between the host and therefore the agent or environment. The presence (or absence) of the agent is important for infection to occur (or be prevented). The environment must support the agent, and therefore the agent must be transmitted to a susceptible host in an appropriate time, manner, and sufficient dose to cause infection and disease. In an experiment, those that are exposed to the agent or putative cause are exposed only because the investigator has assigned the exposure to the topic . Furthermore, the rationale for assigning the precise exposure to the actual subject must be simply the pursuit of the study protocol-that is, the sole reason for the assignment must be to evolve to the protocol instead of to satisfy the requirements of the topic .