任何对身体有害的外来物或生物。一般来说，病原体是微生物，如：细菌、病毒、真菌或寄生虫。每一种情况中，感染生物都用寄主的身体来生存和生长，经常集中于一个特定器官。有时这会影响正常的细胞功能导致疾病。有些细菌还会分泌对寄主有毒的蛋白质导致轻度反应如腹泻，重则会致命。病原体展现出一系列特殊的蛋白质，允许它们感染寄主并在其体内生长；这些蛋白质是治疗的靶子。

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.