Which of the following is an example of an infection?

The invasion of body tissue by bacteria is a clear and established example of an infection. Infections occur when pathogens, such as bacteria, viruses, or fungi, penetrate the body's defenses, begin to multiply, and disrupt normal bodily functions. This invasion often triggers an immune response as the body recognizes these foreign invaders and attempts to combat them. The presence of bacteria in tissue can lead to various symptoms, such as inflammation, pain, fever, and other signs of illness, differentiating it from other bodily processes or states.

The other options represent processes or conditions that do not involve infections. The accumulation of nutrients in the body is related to nutrition and metabolism, whereas the maintenance of hydration levels is important for bodily function but does not imply the presence of pathogens. Similarly, the production of antibodies is a response to infections rather than an infection itself, as antibodies help the body fight against infections but do not constitute an infection on their own.