World Immunisation Week | Part 1 | Basic Terminologies


So, it all starts with health or being healthy. But what is Health?

Way back in 1948, WHO defined health through a phrase –

Health is a state of complete physical, mental and social well-being and not merely the absence of disease or infirmity.

Further, in 1986, WHO added clarification to it by stating –

Health is a resource for everyday life, not the objective of living. It is a positive concept emphasizing social and personal resources, as well as physical capacities, meaning thereby that health is a resource to support an individual’s function in the society at large, rather than restricting to oneself. A healthy lifestyle provides the means to lead a full life with meaning and purpose.

During the first decade of 21st century, in the year 2009, Lancet defined health as

the ability of a body to adapt to new threats and infirmities.

In short, health is a disease-free state. Then the question comes, what is a disease?


A disease is a particular abnormal medical condition that negatively affects the structure or function of all or part of an organism exhibiting specific signs and symptoms.  

A disease may be caused by external factors such as pathogens or by internal dysfunctions of the immune system.

Broadly speaking, disease is referred to any condition that includes injuries, disabilities, disorders, syndromes, infections, isolated symptoms, deviant behaviours, and atypical variations of structure and function.

Likewise, diseases are mainly of four types:

  • infectious diseases (caused by organisms including bacteria, virus, fungi or parasites)
  • deficiency diseases (caused by the lack of certain essential nutrients including vitamins and minerals)
  • hereditary diseases (including both genetic diseases and non-genetic hereditary diseases)
  • physiological diseases (caused by a number of abiotic factors)

Diseases are also classified as either communicable (that spreads from person to person) or non-communicable (that does not spread from person to person).

When our body is invaded by internal and external factors that adversely affect the healthy status then immune system become our defender. Now let us understand about this saviour, which is the most essential component of our body.

Immune system

Immune system, which is the most crucial for our survival, comprises of a vast network of cells, tissues, and organs spread throughout our body. Distinguishing our tissue from the foreign tissue, from self to non-self agents is the most critical role immune system play.

Cells, Tissues and Organs of Immune System

Primary lymphoid organs of the immune system are the thymus and bone marrow, whereas secondary lymphatic tissues are spleen, tonsils, lymph vessels, lymph nodes, adenoids, skin and liver.

Leukocytes (white blood cells) including phagocytes like macrophages, neutrophils and dendritic cells, mast cells, eosinophils, basophils, natural killer cells, and lymphocytes play a significant role in defending our body through immune system.


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