Class 9 Notes

Class 9 Science Notes Chapter 13 (Why do We Fall Ill) – Science Book

Philoid

Philoid

— 6 min read

Science
Alright class, let's focus on Chapter 13, 'Why Do We Fall Ill?'. This is a crucial chapter, not just for understanding biology but also for general awareness, often tested in competitive exams. We need to understand the concepts of health, disease, their causes, and how we can prevent and treat illnesses. Pay close attention to the key terms and distinctions.

Chapter 13: Why Do We Fall Ill? - Detailed Notes for Government Exam Preparation

1. Health and its Significance

  • Health: Defined by the World Health Organization (WHO) as a state of complete physical, mental, and social well-being, and not merely the absence of disease or infirmity.
  • Significance:
    • Personal health is essential for individual happiness and efficiency.
    • Community health is vital for the overall progress and productivity of society.
    • Good health enables us to cope with social and psychological pressures.
  • Factors Affecting Health:
    • Personal Hygiene: Maintaining cleanliness of the body and surroundings.
    • Public Hygiene: Clean environment, proper waste disposal, safe drinking water provided by community efforts.
    • Nutrition: Balanced diet providing necessary energy and nutrients.
    • Social Equality and Harmony: Reduces stress and promotes mental well-being.
    • Economic Conditions: Affect access to nutritious food, clean living conditions, and healthcare.

2. Distinction Between 'Healthy' and 'Disease-Free'

  • Disease-Free: Refers to the absence of any specific discomfort or derangement of body function. One can be disease-free but not necessarily healthy (e.g., a musician unable to perform optimally due to stage fright might be disease-free but not fully healthy in the broader sense).
  • Healthy: Encompasses physical fitness, mental alertness, and social comfort. It's a broader concept than just being free from disease.

3. Disease: What it Means

  • Disease (Dis-ease): Literally means being uncomfortable or disturbed ease. It refers to any condition that impairs the normal functioning of the body or mind.
  • Identifying Disease:
    • Symptoms: Subjective indications of disease that only the patient can feel (e.g., headache, nausea, fatigue, pain). They are general indicators.
    • Signs: Objective indications of disease that can be observed or measured by others, including doctors (e.g., fever, rash, swelling, high blood pressure, results of lab tests). Signs provide more definite clues.

4. Types of Diseases

  • Based on Duration:

    • Acute Diseases: Last for a short period and usually do not cause long-term effects on health (e.g., Common cold, Typhoid, Cholera). Recovery is generally complete.
    • Chronic Diseases: Last for a long time, often a lifetime, and have drastic long-term effects on health (e.g., Tuberculosis (TB), Diabetes, Elephantiasis, Arthritis, Cancer). They cause prolonged ill health.

  • Based on Cause:

    • Infectious Diseases (Communicable Diseases): Caused by external agents (pathogens) and can spread from an infected person to a healthy person (e.g., Common cold, Influenza, TB, Malaria, AIDS).
    • Non-infectious Diseases (Non-Communicable Diseases): Caused by factors other than infectious agents (e.g., genetic abnormalities, lifestyle factors, nutritional deficiencies, organ degeneration). They do not spread from person to person (e.g., High blood pressure, Diabetes, Cancer, Goitre, Scurvy, Arthritis).

5. Causes of Diseases

  • Immediate Causes: The primary factors that directly cause the disease.
    • Infectious Agents (Pathogens): Microorganisms like viruses, bacteria, fungi, protozoa, or multicellular organisms like worms.
  • Contributory Causes: Factors that make an individual more susceptible to disease or contribute to its occurrence.
    • Poor nutrition/unbalanced diet.
    • Genetic differences/predisposition.
    • Lack of public services (clean water, sanitation).
    • Poverty and poor living conditions.
    • Lack of awareness.

6. Infectious Diseases: Agents and Spread

  • Infectious Agents (Pathogens):

    • Viruses: Cause diseases like Common cold, Influenza, Dengue fever, AIDS, Measles, Mumps, Polio, COVID-19. (Viruses live inside host cells).
    • Bacteria: Cause diseases like Typhoid fever, Cholera, Tuberculosis (TB), Anthrax, Tetanus, Bacterial pneumonia. (Bacteria often multiply rapidly).
    • Fungi: Cause many common skin infections like Ringworm, Athlete's foot.
    • Protozoa: Cause diseases like Malaria (Plasmodium), Kala-azar (Leishmania), Amoebic dysentery (Entamoeba histolytica), Sleeping sickness (Trypanosoma).
    • Worms (Helminths): Cause diseases like Elephantiasis (Filarial worms), Ascariasis (Roundworms), Taeniasis (Tapeworms).

  • Means of Spread (Transmission):

    • Through Air (Air-borne): Pathogens transmitted via droplets from coughing, sneezing (e.g., Common cold, Pneumonia, Tuberculosis).
    • Through Water (Water-borne): Pathogens transmitted via contaminated water (e.g., Cholera, Amoebic dysentery, Typhoid).
    • Through Physical Contact:
      • Direct skin contact (e.g., Fungal infections, Scabies).
      • Sexual contact (e.g., Syphilis, Gonorrhoea, AIDS/HIV). HIV can also spread via blood transfusion and from mother to child during pregnancy/breastfeeding.
    • Through Vectors: Animals that carry infectious agents from a sick person to a healthy person without getting sick themselves.
      • Mosquitoes: Anopheles (Malaria), Aedes (Dengue).
      • Flies: Can carry pathogens from garbage/excreta to food (e.g., Cholera, Typhoid).

7. Organ-Specific and Tissue-Specific Manifestations

  • Pathogens enter the body through different routes (nose, mouth, skin, etc.) and often target specific organs or tissues where conditions are favourable for their growth.
  • Examples:
    • Lungs: Pathogens entering via nose (e.g., Tuberculosis bacteria). Symptoms: Cough, breathlessness.
    • Gut/Liver: Pathogens entering via mouth (e.g., Typhoid bacteria, Jaundice virus). Symptoms: Stomach ache, vomiting, jaundice.
    • Brain: Pathogens reaching via bloodstream or nerves (e.g., Japanese encephalitis virus, Rabies virus). Symptoms: Headache, vomiting, fits, unconsciousness.
    • Lymph nodes/Immune System: HIV attacks the immune system itself, weakening the body's defenses.
  • The signs and symptoms of a disease often depend on the target organ/tissue. General effects like inflammation (swelling, pain) and fever are common immune responses.

8. Principles of Treatment

  • Two main approaches:

    • 1. Symptomatic Treatment (Reduce effects): Providing treatment to reduce symptoms like fever, pain, or loose motions. This provides relief and allows the body to rest and conserve energy to fight the infection. It doesn't cure the disease itself.
    • 2. Causal Treatment (Kill the cause): Killing the pathogen using specific medicines.
      • Antibiotics: Effective against bacteria. They block essential biochemical pathways specific to bacteria (e.g., cell wall synthesis) without harming human cells (which lack these pathways). Examples: Penicillin.
      • Antivirals: Difficult to make because viruses use the host cell's machinery. Drugs targeting viral replication pathways exist but are fewer and often specific.
      • Antifungals: Target fungal-specific pathways.
      • Antiprotozoals: Target protozoan-specific pathways.
      • Antihelminthics: Target worms.

  • Why antibiotics don't work on viruses: Viruses lack the specific biochemical pathways (like cell wall synthesis) that antibiotics target. Taking antibiotics for viral infections is ineffective and can contribute to antibiotic resistance.

9. Principles of Prevention

  • Prevention is better than cure, especially for infectious diseases.
  • Two main approaches:
    • 1. General Ways (Preventing Exposure):
      • Maintaining public hygiene (sanitation, clean water, waste disposal).
      • Maintaining personal hygiene.
      • Avoiding overcrowded places during outbreaks.
      • Using mosquito nets/repellents to avoid vector-borne diseases.
      • Consuming safe food and water.
      • Having adequate nutrition to boost the immune system.
    • 2. Specific Ways (Strengthening the Immune System):
      • Immunisation/Vaccination: Introducing a weakened, killed, or specific part of a pathogen (antigen) into the body. This 'fools' the immune system into producing specific antibodies and memory cells without causing the actual disease. If the actual pathogen enters later, the immune system mounts a quick and strong response, preventing the disease.
      • Immune System: A complex network of cells and proteins that defends the body against infection. It recognizes foreign invaders (pathogens) and eliminates them. Vaccination enhances this natural defence mechanism.
      • Vaccines Available: For diseases like Tetanus, Diphtheria, Whooping cough, Measles, Mumps, Rubella (MMR), Polio, Hepatitis B, Tuberculosis (BCG), COVID-19.

10. Community Health

  • Personal health is dependent on community health. Many preventive measures (clean water, sanitation, vaccination programs) require collective community and government action.

Multiple Choice Questions (MCQs)

  1. According to WHO, health is a state of complete well-being in which aspects?
    (a) Physical and Mental only
    (b) Physical and Social only
    (c) Physical, Mental, and Social
    (d) Absence of disease only

  2. Which of the following is an example of a chronic disease?
    (a) Common Cold
    (b) Cholera
    (c) Elephantiasis
    (d) Typhoid

  3. Antibiotics are generally effective against diseases caused by:
    (a) Viruses
    (b) Bacteria
    (c) Fungi
    (d) Protozoa

  4. Kala-azar is caused by which type of infectious agent?
    (a) Virus
    (b) Bacterium
    (c) Fungus
    (d) Protozoa (Leishmania)

  5. Which of the following diseases is NOT transmitted by mosquitoes?
    (a) Dengue
    (b) Malaria
    (c) Japanese Encephalitis
    (d) Cholera

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

  7. Which of the following is a non-infectious disease?
    (a) Tuberculosis
    (b) Influenza
    (c) High Blood Pressure
    (d) AIDS

  8. Diseases that last for only very short periods are called:
    (a) Chronic diseases
    (b) Acute diseases
    (c) Congenital diseases
    (d) Non-infectious diseases

  9. Which of the following is NOT a general way of preventing infectious diseases?
    (a) Living in hygienic conditions
    (b) Taking antibiotics regularly
    (c) Avoiding exposure to infected persons
    (d) Ensuring availability of safe drinking water

  10. AIDS (Acquired Immuno Deficiency Syndrome) is caused by a:
    (a) Bacterium
    (b) Protozoan
    (c) Virus (HIV)
    (d) Fungus

Answer Key:

  1. (c)
  2. (c)
  3. (b)
  4. (d)
  5. (d)
  6. (b)
  7. (c)
  8. (b)
  9. (b)
  10. (c)

Study these notes carefully. Understanding the difference between infectious and non-infectious diseases, the types of pathogens, modes of transmission, and the principles of treatment and prevention is key. Remember the specific examples given for each category. Good luck with your preparation!

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