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| Introduction |
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| Living organisms are exposed to various external or foreign substances and disease causing organisms like bacteria, viruses and fungi. Such organisms which can cause a disease are known as Pathogens. It is observed that some human beings are more prone to a certain disease and some others are immune to it. |
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| For e.g., if a person had suffered from measles once in life, he/she is not attacked by the same disease again. This means that the person has become immune to that specific disease/infection. |
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| The ability to ward off disease is called resistance. Lack of resistance is called susceptibility. |
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| The system of animal body, which protects it from various infections is called the Immune System. Body's defence against diseases is called Immunity. The study of the Immune system is known as immunology. |
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