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Infections that enter the body through external portals are classified as exogenous infections. This classification is based on the source of the infection. Exogenous infections originate from external sources, meaning the pathogens are introduced from outside the body, such as through the skin, mucous membranes, or direct environmental exposure. This contrasts with endogenous infections, which arise from microorganisms that are already present in the body, often from normal flora that become pathogenic under certain conditions.

Opportunistic infections refer to infections caused by pathogens that typically do not cause disease in healthy individuals but can affect those with weakened immune systems. Nosocomial infections, also known as hospital-acquired infections, specifically refer to infections acquired during a hospital stay, regardless of the infection source. Hence, the classification as exogenous accurately depicts the mode of entry for pathogens that come from external environments.