任何對身體有害的外來物或生物。一般來說，病原體是微生物，如：細菌、病毒、真菌或寄生蟲。每一種情況中，感染生物都用寄主的身體來生存和生長，經常集中于一個特定器官。有時這會影響正常的細胞功能導致疾病。有些細菌還會分泌對寄主有毒的蛋白質導致輕度反應如腹瀉，重則會致命。病原體展現出一系列特殊的蛋白質，允許它們感染寄主并在其體內生長；這些蛋白質是治療的靶子。

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.