Define the term 'pathogen.'

The term 'pathogen' is defined as an organism that causes disease. This includes a variety of infectious agents such as bacteria, viruses, fungi, and parasites. When these organisms invade the body, they can disrupt normal bodily functions, leading to illness. Each of these pathogens has specific mechanisms for causing disease, which may involve destroying host cells, producing toxins, or triggering inflammatory responses.

Understanding what constitutes a pathogen is fundamental in immunology, as it helps in developing vaccines, treatments, and understanding immune responses. The focus is on their ability to cause damage and provoke an immune response, setting the stage for how the body reacts to infection and how health can be restored.