Pathogen

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Pathogen (Greek: παθογένεια, "cause of suffering") is a biological agent that causes disease in its host. Another term of a pathogen is a parasitic microorganism. Generally this term is given to agents that disrupt the normal physiology of animals or multicellular plants. However, pathogens can also infect unicellular organisms from all the biological kingdoms.

Generally, only highly pathogenic organisms can cause disease, while the rest rarely cause disease. Opportunistic pathogens are rare pathogens that cause disease in immunocompetent people but can cause serious illness / infection in immunocompromised individuals. 


This opportunistic pathogen is generally a member of the normal flora of the body. The term opportunist itself refers to the ability of an organism to take advantage of the opportunity provided by the decline of the host defense system to cause illness.

Generally, all pathogens have been outside the cells of the body with a certain time span (extracellular) when they are exposed by antibody mechanisms, but when the pathogen enters an intracellular phase unattainable by the antibody, T cells will play its role.