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Disease is rarely caused by a single factor. Most diseases result from a complex interaction between the individual, the biological agents that produce illness, the physical and social environment, and the conditions in which people live and work. The inter-relationship between health and environment is therefore fundamental to understanding disease causation. Health is influenced not only by genetic and biological characteristics but also by environmental quality, nutrition, socioeconomic conditions, occupation, lifestyle, sanitation, and access to health services. The concept of multiple causation recognizes that several factors may act together to initiate, promote, or aggravate disease. The agent-host-environment model provides a basic framework for understanding disease causation. The agent may be a biological organism such as a bacterium, virus, fungus, parasite, or protozoan; a chemical substance such as a toxic metal, pesticide, or pollutant; or a physical factor such as radiation, heat, noise, or trauma. The host is the person or population exposed to the agent and possesses characteristics that influence susceptibility. The environment includes all external conditions that affect both the agent and the host. Disease develops when an appropriate agent comes into contact with a susceptible host under environmental conditions that favour its occurrence. Biological factors are important determinants of disease. Infectious diseases are caused by microorganisms and parasites, but the presence of an infectious agent alone does not necessarily produce disease. The dose or number of organisms, their virulence, route of entry, and ability to survive in the environment influence transmission. For example, contaminated drinking water may contain pathogens, but disease occurrence also depends on the quantity consumed, the immunity of the individual, nutritional status, and general living conditions. Similarly, respiratory infections are influenced by the characteristics of the infectious organism as well as crowding, ventilation, temperature, and the immune status of exposed individuals. Host factors strongly influence the development and severity of disease. Age is an important determinant because infants and older adults may be _________ ______ __________ ____ __________ ___ _____ ________ ______.
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