Page 1 - 2. Lesson Notes- Infectious Disease
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SAI International School
Subject – Biology, Class- IX
Chapter-Why do we fall ill?
Subtopic- Infectious Diseases
Module -21
Lesson Notes
Infectious diseases
Infectious diseases are the disease that is caused due to the entry of harmful
microorganisms into the body. i.e.- pathogens.
Infectious agents:
Infectious agents are all around us, and they come in different shapes and sizes. They
can be categorized based on some common characteristics.
1. Some are single-celled organisms such as fungi, bacteria, and viruses.
2. Others are multicellular organisms such as worms.
