Which statement best defines 'infection' in a medical context?

In a medical context, the definition of 'infection' is best captured by the concept of the invasion and reproduction of pathogens within the body's tissues. This encompasses a few critical aspects. First, it highlights that infections are caused by pathogens, which include bacteria, viruses, fungi, and parasites. These organisms invade a host's body, manage to enter tissues, and begin to reproduce, thereby establishing their presence. This process can lead to disease if the host's immune response is unable to contain the pathogens.

While other options touch on related concepts, such as the body's immune response or the mere presence of pathogens, they do not adequately convey the essence of an infection, which fundamentally involves invasion and reproduction of microorganisms that can lead to a pathological state. Thus, the correct answer captures the dynamic nature of infections and their potential to cause illness.