Define Pathogen , How they adapted in body of host

• Disease is a condition in which the function of the body or part of the body is disturbed. Any deviation in normal structure and function in the body is called disease.

• Disease is characterized by the appearance of symptoms in the body. The causative agent of disease is called pathogen. The establishment of pathogens inside the body of an organism is termed infection.

Define Pathogen , How they adapted in body of host

• A number of organisms may act as pathogen. The usual place where these pathogens remain present is called reservoir of infection.

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• These pathogens are transmitted either directly or indirectly to spread the infection.

• Contact transmission, droplet infections , contact with contaminated soil etc. are modes of direct infections whereas Transmission through vectors, Air, water etc are modes of indirect transmission. 

• The organism that causes disease must make their entry through a particular pathway. This is known as a portal of entry.

• The main portal of entry are skin, respiratory tract, digestive tract, urogenital tract etc. 

Read about more about the Immunity : Active & Passive immunity 

• Certain organism causes disease only if they enter the body through a specific paths. For ex-Typhoid causative agent causes typhoid when it makes entry through the mouth by contaminated food and water. 

• After making an entry inside the body, The pathogen releases several kinds of chemical substances like toxins. 

• These toxins cause physiological, morphological and anatomical changes in the body of the host. 

• The toxins released by the pathogen can kill our protective cell and the pathogen gets an opportunity to grow in the body of the host. 

• All organisms have resistance power against the pathogen. 

Read more about the Protein needed against the infection : Monoclonal antibody and Interferon 

• Besides this, the host body also has defense barriers that prevent the entry of pathogens. Skin is the first defense structure and act as physical barrier against entry of microorganism. 

• The conjunctiva and oral cavity releases chemicals like lysozyme in tears and saliva that also prevent or check the entry of pathogen.

• Maintenance of acidic environment in the stomach and vagina also control the entry of pathogen. 

• When above defense barrier are fail to prevent the entry of pathogen than our body cells destroy these pathogen by the process of phagocytosis.

• Macrophages and neutrophils are promoting cell that carry out this function. 

• Viral infections induce body cells to release antiviral substances called interferon which inhibit the viral multiplication inside the body.