Communicable
diseases
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Communicable diseases are those that spread by an infectious agent, such as bacteria, viruses, fungi or parasites. Most of these diseases can be passed from person to person so the words “contagious” or “infectious” are often used when talking about communicable diseases.
Dynamics/process of disease transmission ( how the communicable disease spread)
To complete the process of disease transmission it needs five links. These are- source or reservoir, portal of exit, mode of transmission, portal of entry and susceptible host. These are known as chain of infection.
1. Source or reservoir- the source is defined as the person, animal object or substance from which and infectious agent passes to the host. Reservoir is defined as the person, animal object or substance in which the infectious agent live and multiply before passing the susceptible host. Reservoir may be human reservoir (case or carrier) Animal reservoir (animal and birds) and nonliving reservoir such as soil.
2. Portal of exit- Portal of exit is the path by which a pathogen leaves its host. The portal of exit usually corresponds to the site where the pathogen is localized. For example, influenza viruses and Mycobacterium tuberculosis exit the respiratory tract.
3. Mode of transmission- mode of transmission are the different ways by which infectious agent is transmitted to the host. It is classified as direct transmission and indirect transmission.
a.) Direct transmission – when infectious agent is directly transferred from source to host than it is known as direct transmission. It includes direct contact, droplet infection, contact with soil, inoculation into skin and trans placental transmission.
b.) Indirect transmission- when infectious agent is not directly transferred from source to host than it is known as indirect transmission. It includes vehicle born infection, vector born infection airborne infection, fomite born infection and infection through contaminated hand and fingers.
4. Portal of entry- The portal of entry refers to the manner in which a pathogen enters a susceptible host. The portal of entry must provide access to tissues in which the pathogen can multiply or a toxin can act. Often, infectious agents use the same portal to enter a new host that they used to exit the source host. For example, influenza virus exits the respiratory tract of the source host and enters the respiratory tract of the new host.
5. Susceptible host- The final link in the chain of infection is a susceptible host. He is a person who is susceptible to get infection. Susceptibility of a host depends on specific immunity, and nonspecific factors that affect an individual’s ability to resist infection.
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