The term pathogen came into practice in the 1880s. The term is most often used for agents that disrupt the normal physiology of a multicellular animal or plant. Nevertheless, pathogens can infect unicellular organisms from all of the biological kingdoms.
In biology, a pathogen can be described as anything that can produce disease. The human body is naturally full of various microbes. But, these microbes only create a problem if your immune system is not strong or if the microbes manage to enter a normally sterile part of your body. A pathogen can also be referred to as an infectious agent, or simply a germ.