Class 9 Science Notes Chapter 17 (Chapter 17) – Examplar Problem (English) Book

Alright class, let's get straight into Chapter 17, which deals with 'Why Do We Fall Ill?'. This is a crucial chapter, not just for your exams but for understanding health in general, which often features in government exam questions under General Science or Awareness. We'll focus on the key concepts you need to know.

(Note: This assumes Chapter 17 refers to 'Why Do We Fall Ill?' based on the standard NCERT Class 9 Science sequence.)

Chapter 17: Why Do We Fall Ill? - Detailed Notes for Government Exams

1. Concept of Health

• Definition: Health is more than just the absence of disease. The World Health Organization (WHO) defines it as "a state of complete physical, mental, and social well-being and not merely the absence of disease or infirmity."
• Factors Influencing Health:
  • Personal Factors: Balanced diet, personal hygiene, exercise, adequate sleep, absence of addictions.
  • Community Factors: Clean environment (sanitation, waste disposal, clean water), availability of nutritious food, social harmony, economic conditions.
  • Public Health Infrastructure: Access to healthcare facilities, vaccination programs, disease surveillance.
• Distinction: Being 'disease-free' is not the same as being 'healthy'. A person might not have a specific disease but could still be unhealthy due to stress (mental well-being) or inability to cope with social situations (social well-being).

2. Concept of Disease

• Definition: Disease literally means 'disturbed ease'. It's a condition that impairs the normal functioning of the body or mind, causing discomfort, dysfunction, or distress.
• Symptoms vs. Signs:
  • Symptoms: Subjective indications of disease felt by the patient (e.g., headache, nausea, fatigue, pain).
  • Signs: Objective evidence of disease that can be observed or measured by others (e.g., fever, rash, swelling, high blood pressure).

3. Causes of Diseases

• Diseases arise from multiple causes acting at different levels:
  • Immediate Causes (Primary): The direct agent causing the disease, usually a pathogen (microbe) or a deficiency.
  • Contributory Causes (Secondary/Tertiary): Factors that make an individual susceptible, like poor nutrition, genetic predisposition, weak immune system, poor sanitation, poverty, lack of public services.

4. Types of Diseases

• Based on Duration:
  • Acute Diseases: Short duration, sudden onset, effects are usually temporary, patient generally recovers completely. Examples: Common cold, flu, cholera, typhoid.
  • Chronic Diseases: Long duration (months, years, or lifetime), develop gradually, cause long-term damage or debility. Examples: Tuberculosis (TB), diabetes, hypertension, arthritis, elephantiasis, cancer. Chronic diseases have drastic long-term effects on health compared to acute diseases.
• Based on Cause/Transmission:
  • Infectious Diseases (Communicable): Caused by external pathogens (microorganisms like bacteria, viruses, fungi, protozoa, worms). Can spread from an infected person to a healthy person. Examples: Common cold, TB, malaria, AIDS, cholera.
  • Non-infectious Diseases (Non-communicable): Not caused by pathogens and cannot spread from person to person. Often caused by internal factors, lifestyle choices, environmental factors, genetic disorders, or deficiencies. Examples: Cancer, diabetes, high blood pressure, heart disease, arthritis, genetic disorders (like haemophilia), deficiency diseases (like scurvy, pellagra).

5. Infectious Agents (Pathogens)

• These are disease-causing organisms. Key groups include:
  • Viruses: Cause diseases like common cold, influenza, measles, mumps, chickenpox, polio, hepatitis, dengue, AIDS (caused by HIV). Viruses need host cells to replicate.
  • Bacteria: Cause diseases like typhoid, cholera, tuberculosis, tetanus, anthrax, diphtheria, whooping cough, bacterial pneumonia, acne.
  • Fungi: Cause skin infections like ringworm, athlete's foot.
  • Protozoa: Cause diseases like malaria (Plasmodium), amoebic dysentery (Entamoeba histolytica), kala-azar (Leishmania), sleeping sickness (Trypanosoma).
  • Worms (Helminths): Cause intestinal infections (like Ascariasis by roundworm Ascaris, Taeniasis by tapeworm) and diseases like elephantiasis/filariasis (by filarial worms).
• Antibiotics: Drugs effective against bacteria by blocking essential biochemical pathways (like cell wall synthesis). They are ineffective against viruses because viruses use the host's machinery and lack these specific pathways. Developing effective antiviral drugs is challenging.

6. Means of Spread (Modes of Transmission of Infectious Diseases)

• Air-borne: Through droplets from coughing, sneezing, talking. Pathogens spread via air. Examples: Common cold, influenza, pneumonia, tuberculosis.
• Water-borne: Through contaminated drinking water (often fecal-oral route). Examples: Cholera, typhoid, amoebiasis, hepatitis A.
• Vector-borne: Through intermediate organisms (vectors) that carry pathogens.
  • Mosquitoes: Female Anopheles transmits malaria; Female Aedes transmits dengue and chikungunya.
  • Houseflies: Can transfer pathogens from waste/excreta to food.
• Contact-borne:
  • Direct Physical Contact: Skin-to-skin (e.g., scabies, ringworm), sexual contact (e.g., syphilis, gonorrhoea, AIDS).
  • Indirect Contact: Through contaminated objects (fomites) like clothes, utensils, bedding.
• Spread of AIDS (HIV): Primarily through sexual contact, blood transfusions (infected blood), sharing infected needles, and from infected mother to child (during pregnancy, birth, or breastfeeding). Not spread by casual contact (hugging, sharing meals, shaking hands).

7. Organ-specific and Tissue-specific Manifestations

• Pathogens often target specific organs or tissues based on their mode of entry and adaptation.
  • Entry via nose -> Lungs (e.g., TB bacteria).
  • Entry via mouth -> Gut lining (e.g., typhoid bacteria, Entamoeba) or Liver (e.g., Hepatitis virus).
  • Entry via mosquito bite -> Bloodstream -> Liver/RBCs (Malaria protozoa) or other tissues.
  • Entry into brain (e.g., Japanese encephalitis virus).
• Inflammation: A common response of the immune system to infection or injury, characterized by redness, swelling, heat, pain, and sometimes fever. It's an attempt to localize and eliminate the harmful agent.

8. Principles of Treatment

• Two main strategies:
  • 1. Symptomatic Treatment: Aims to reduce or manage the symptoms (e.g., medicine for fever, pain relief, bed rest). Provides relief but doesn't eliminate the pathogen.
  • 2. Etiological Treatment (Targeting the Cause): Aims to kill the pathogen using specific drugs (e.g., antibiotics for bacteria, antivirals for viruses, antifungals for fungi, antiprotozoals for protozoa). This is essential for curing the disease.

9. Principles of Prevention

• Prevention is crucial as treatment has limitations (body function damage may not be fully reversible, treatment takes time/resources, infected person can spread disease).
• Two main strategies:
  • 1. General Ways (Preventing Exposure): Applicable to many infectious diseases.
    • Maintaining public and personal hygiene (handwashing, sanitation).
    • Ensuring safe drinking water and food.
    • Avoiding overcrowded places (for air-borne diseases).
    • Vector control (preventing mosquito breeding, using nets/repellents).
    • Eating a balanced diet to maintain a strong immune system.
  • 2. Specific Ways (Strengthening the Immune System):
    • Immunisation / Vaccination: Introducing harmless forms (killed, weakened, or parts) of a pathogen (vaccine) into the body. This 'tricks' the immune system into developing specific antibodies and memory cells, providing immunity against future infection by the actual pathogen.
    • Examples of vaccine-preventable diseases: Polio, Tetanus, Diphtheria, Pertussis (Whooping Cough), Measles, Mumps, Rubella, Hepatitis B, Tuberculosis (BCG vaccine). Public health programs focus on universal immunization.

10. Immune System

• The body's defence mechanism against pathogens. Involves specialized cells (like lymphocytes) that identify, attack, and eliminate foreign invaders.
• It has 'memory' – remembers previous encounters with specific pathogens, allowing for a faster and stronger response upon re-exposure (this is the basis of immunity from vaccination or prior infection).
• AIDS (Acquired Immuno Deficiency Syndrome), caused by HIV (Human Immunodeficiency Virus), severely damages the immune system, making the individual highly susceptible to otherwise minor infections (opportunistic infections).

Multiple Choice Questions (MCQs)

1. Which of the following best describes the WHO definition of health?
   a) Absence of physical disease only
   b) Complete physical and mental well-being
   c) State of complete physical, mental, and social well-being
   d) Ability to perform daily tasks efficiently

2. Elephantiasis is a chronic disease caused by:
   a) Bacteria
   b) Virus
   c) Protozoa
   d) Worm (Filarial worm)

3. Which of the following diseases is NOT transmitted by a vector?
   a) Malaria
   b) Dengue
   c) Cholera
   d) Kala-azar

4. Antibiotics are ineffective against the common cold because it is caused by:
   a) Bacteria
   b) Fungus
   c) Virus
   d) Protozoa

5. Which of the following is a non-infectious disease?
   a) Tuberculosis
   b) Influenza
   c) High blood pressure
   d) Measles

6. The principle of vaccination is based on stimulating which property of the immune system?
   a) Inflammation
   b) Phagocytosis
   c) Memory
   d) Antibody production only during infection

7. Which mode of transmission is most common for Tuberculosis (TB)?
   a) Contaminated water
   b) Vector (mosquito)
   c) Air-borne droplets
   d) Sexual contact

8. Treating only the symptoms of a disease, like taking medicine for fever, is called:
   a) Etiological treatment
   b) Preventive treatment
   c) Symptomatic treatment
   d) Immunisation

9. Which pathogen causes Kala-azar?
   a) Plasmodium
   b) Leishmania
   c) Trypanosoma
   d) Entamoeba

10. Maintaining clean surroundings and preventing water stagnation helps primarily in controlling the spread of:
    a) Air-borne diseases
    b) Water-borne diseases
    c) Vector-borne diseases
    d) Deficiency diseases

Answer Key for MCQs:

1. c) State of complete physical, mental, and social well-being
2. d) Worm (Filarial worm)
3. c) Cholera (It's water-borne)
4. c) Virus
5. c) High blood pressure
6. c) Memory
7. c) Air-borne droplets
8. c) Symptomatic treatment
9. b) Leishmania
10. c) Vector-borne diseases (especially mosquito-borne)

Make sure you understand the differences between acute/chronic and infectious/non-infectious diseases, know examples of diseases caused by different pathogens, their modes of transmission, and the principles of prevention, especially vaccination. Good luck with your preparation!