Any foreign agent or organism that is harmful to the body. Typically, pathogens are microscopic organisms such as bacteria, viruses, fungi or parasites such as worms. In each case, the infecting organism uses the host body to live and grow, and is often restricted to a particular organ. Sometimes this affects normal cellular functions, leading to illness. Some bacteria also secrete proteins that are toxic for the host, leading to mild effects such as diarrhea, or to fatal effects. Pathogens exhibit a unique set of proteins, which allow them to infect and grow in their hosts; these proteins are the target of therapeutic intervention.