Hamid Reza Edraki
Typically the term is used to describe an infectious microorganism or agent such as a virus, bacterium, protozoan, prion, viroid, or fungus. Small animals such as certain worms or insects can also cause or transmit disease However these animals are usually in common parlance referred to as parasites rather than pathogens. The scientific study of microscopic organisms including microscopic pathogenic organisms is called microbiology, while parasitology refers to the scientific study of parasites and the organisms that host them. There are several pathways through which pathogens can invade a host. The principal pathways have different episodic time frames but the soil has the longest or most persistent potential for harboring a pathogen. Diseases in humans that are caused by infectious agents are known as pathogenic diseases. Not all diseases are caused by pathogens other causes are for example toxins genetic disorders and the hosts own immune system.Pathogenicity is the potential disease-causing capacity of pathogens. By this standard an organism may be said to be pathogenic or non-pathogenic in a particular context but not more pathogenic than another. Such comparisons are described instead in terms of relative virulence. Pathogenicity is also distinct from the transmissibility of a virus, which quantifies the risk of infection. A pathogen may be described in terms of its ability to produce toxins, enter tissue, colonize, hijack nutrients, and its ability to immunosuppress the host
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