Why Do We Fall Ill

NCERT Class 9 Science, Chapter – Why Do We Fall Ill?, from the Science Textbook, explains the concepts of health, disease, and the factors that affect human well-being. The chapter helps students understand the difference between being healthy and being disease-free, the causes of diseases, and the importance of maintaining good health through proper nutrition, hygiene, and preventive measures.


The chapter describes health as a state of complete physical, mental, and social well-being. It explains that good health depends not only on personal habits but also on community factors such as clean surroundings, safe drinking water, proper sanitation, and social equality. The chapter further discusses different types of diseases, including acute and chronic diseases, and highlights how poor health can result from both infectious and non-infectious causes.

The chapter also focuses on infectious diseases, their causes, modes of transmission, treatment, and prevention. Students learn how disease-causing microorganisms such as viruses, bacteria, fungi, and protozoa spread through air, water, physical contact, and vectors like mosquitoes. It emphasizes that prevention is better than cure and explains the role of public hygiene, proper nutrition, and immunization in protecting individuals and communities from diseases.