What does the establishment and growth of microorganisms on or in a host define?

The establishment and growth of microorganisms on or in a host is defined as infection. This term implies that the microorganisms not only appear in the host but actively multiply and cause harm or disease. An infection occurs when the presence of these pathogens leads to an immune system response, which can result in various symptoms and clinical manifestations.

While other terms are relevant to the context of microorganisms and hosts, they do not capture the complete idea of infectious disease. Contamination refers merely to the presence of microorganisms without implying any growth or damage to the host. Invasion typically describes a more aggressive process where pathogens breach barriers into sterile areas of the host, which may or may not lead to infection. Colonization, while indicating that organisms are present and growing, does not always result in harmful effects or clinical symptoms, which is a crucial aspect of infection. Thus, in the context of this question, infection is the term that best describes the process of microorganisms establishing themselves and causing an impact on the host.