任何對(duì)身體有害的外來(lái)物或生物。一般來(lái)說，病原體是微生物，如：細(xì)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(lái)生存和生長(zhǎng)，經(jīng)常集中于一個(gè)特定器官。有時(shí)這會(huì)影響正常的細(xì)胞功能導(dǎo)致疾病。有些細(xì)菌還會(huì)分泌對(duì)寄主有毒的蛋白質(zhì)導(dǎo)致輕度反應(yīng)如腹瀉，重則會(huì)致命。病原體展現(xiàn)出一系列特殊的蛋白質(zhì)，允許它們感染寄主并在其體內(nèi)生長(zhǎng)；這些蛋白質(zhì)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.