What term refers to the invasion of body tissues by disease-causing pathogens?

The term that refers to the invasion of body tissues by disease-causing pathogens is "infection." An infection occurs when pathogens, such as bacteria, viruses, fungi, or parasites, enter the body and begin to multiply, leading to illness or disease. This biological process often triggers the body's immune response as it tries to combat the invading pathogens.

In contrast, contamination refers to the presence of pathogens or impurities on surfaces or objects, but it does not necessarily imply that the pathogens have invaded or caused an illness within the body. Diseases refer to a broader range of health conditions, which can result from infections but may also arise from non-infectious causes. Infestation specifically refers to the invasion of the body by larger parasites, such as worms or insects, rather than microbial pathogens. Therefore, infection is the most appropriate term to describe the scenario given in the question.