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How does the epidemiologic triad explain the occurrence of disease?

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Know the investigations of researchers using genomic information to reconstruct the cause and transmission routes of the bubonic plague and the Black Death — epidemiology Researchers using genomic information to trace the transmission routes in past epidemics of plague.

The epidemiologic triad is a foundational model used to understand the occurrence of disease by examining the interaction between three core components: the agent, the host, and the environment.1

Agent: The specific causative factor, such as a pathogen (e.g., bacteria or viruses), whose presence is essential for the disease to occur.1
Host: The organism—such as a human or animal—that harbors the disease. Susceptibility is influenced by factors like age, sex, physiological state, and previous immunity.1
Environment: The external circumstances that allow the agent and host to interact. This includes geographical features, climate, and human-induced changes to the ecosystem.1

Disease occurs when there is an imbalance among these three factors. For example, an epidemic often arises when an ecological shift—such as habitat destruction or poor sanitation—alters the environment, facilitating the transmission of an agent to a susceptible host. By identifying these interactions, epidemiologists can determine the causes of disease and develop strategies to control outbreaks by modifying the environment or host conditions.12

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