Cohort studies look at certain exposures over a

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Cohort studies look at certain exposures over a period of time (prospectively) and how they are related to certain outcomes. Subjects are followed up over a period of time. Case control studies look at certain outcomes and try to establish what might have caused them in the past. For example people suffering from a skin disease may be asked to recall where they have worked or lived previously so as to understand what they had been exposed to and that might have caused the disease. They are also referred to as retrospective studies. Experimental Studies In experimental studies, you set up experiments with controls in order to see the cause and effect. For example, you can decide to give malaria prophylaxis or mosquito nets to one community during the rainy season and deny the same to another similar community. You then observe the two to see if they have the same incidence of malaria or if they have different incidence of malaria. The Epidemiological Triad The triad describes the relationship between various factors that cause disease and how they interact to determine disease occurrence. The factors, namely agent and host, interact in the environment to cause disease in humans. The natural progression of a disease is influenced by the following factors: The causative agent(s) The susceptible host The environment The vector(s) in some diseases This is illustrated in the graphic opposite.
Bishop’s Computers Page 24 of 267 Bishop’s Computers The agent is the harmful aetiological factor that causes health problems. The aetiological factor (agent) may cause disease either by its presence or by its absence. The causative agent may be a substance, living or non-living, excessively present or lacking, tangible or non-tangible. Physical Agents These include mechanical forces such as friction, extreme heat, cold, humidity, pressure, sound, radiation, electricity, etc. Biological Agents Living organisms, such as bacteria, fungi, chlamydia, rickettsiae, protozoa, viruses, mycoplasma, helminthes. Chemical Agents Endogenous chemical agents are those which are made by the body as a result of abnormal metabolism, for example urea (uraemia), serum bilirubin (jaundice), ketones (ketosis, keto- acidosis), uric acid (gout), calcium carbonate(renal stones), etc. Exogenous chemical agents arise from outside the body, for example allergens, meals, fumes, gases, insecticides, etc. Exogenous chemical agents may enter into the body tissues through inhalation, ingestion or inoculation. Genetic Agents Transmitted from parent to child through genes. Nutrient Agents Excessive or deficient intake of nutrients. This results in malnutrition related diseases such as the most common Protein Energy Malnutrition (PEM), for example kwashiorkor and marusmus.

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