Pathogens are microbes that can cause disease including human disease.
Primary Pathogens cause disease by attaching to normal epithelial cells, for example in the intestinal tract, of healthy individuals. These microbes can interfere with normal human functions or they produce harmful toxins that cause human disease. They can also invade and damage body tissues, or spread via the blood or the lymphatic system to cause systemic or distant infection.
Secondary Pathogens do not invade healthy tissues, but they can invade when there is prior damage, for example during a viral infection, insertion of a foreign body or damage caused by a tumour, or as an outcome of a depressed immune response.
Opportunistic pathogens are secondary pathogens that are either part of the normal human flora (commensals or members of the human microbiome), or they exist in the environment. They do not cause infection in healthy subjects, but they can cause severe infections in the immune-compromised.
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