Pathogens are biological agents capable of causing disease in a living organism. This term encompasses a wide range of infectious agents, including bacteria, viruses, fungi, and protozoa, which invade host tissues and disrupt normal physiological processes. The ability of a pathogen to establish infection and cause illness depends on several factors, including its virulence, the host's immune response, and the degree of interaction between the invading agent and the host's natural defenses. Disease causation involves complex biological mechanisms, ranging from direct toxin production to the interference with cellular function, leading to signs and symptoms of illness.