Final answer:
An infection is the colonization of a host by a pathogen, which can be local, systemic, or lead to secondary infections if host defenses are compromised.
Step-by-step explanation:
An infection occurs when a pathogen such as a virus, bacterium, or fungus successfully enters and multiplies within a host's body. The nature and spread of an infection can vary, leading to its classification as either local, focal, or systemic. A local infection is contained within a small area, such as a boil caused by Staphylococcus aureus. On the other hand, a systemic infection is disseminated throughout the body, such as the varicella-zoster virus that causes chickenpox evidenced by widespread skin lesions. Importantly, primary infections can weaken host defenses, making one susceptible to secondary infections which may be more severe and could be caused by opportunistic pathogens or result from the use of broad-spectrum antibiotics that disrupt normal microbiota.
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