Why Do We Fall Ill Class 9 Notes | CBSE Science Chapter 13

Key Concepts: Must Remember

• Health is a state of complete physical, mental, and social well-being, not merely the absence of disease.
• Disease implies 'disturbed ease' – a malfunction of the body or mind, leading to specific symptoms.
• Diseases can be broadly classified as Acute (short duration) or Chronic (long duration, prolonged effects).
• Infectious diseases are caused by external agents (pathogens) and can spread from one person to another.
• Non-infectious diseases are not spreadable and are caused by internal factors like genetics, lifestyle, or environmental issues.
• Pathogens include viruses, bacteria, fungi, protozoans, and worms, each causing distinct diseases.
• Modes of disease spread include air, water, sexual contact, vectors (e.g., mosquitoes), and direct contact.
• Prevention is better than cure – it involves both general (hygiene, nutrition, public sanitation) and specific (immunization) approaches.
• The immune system plays a crucial role in fighting off pathogens and building resistance.
• Antibiotics are effective against bacteria but not viruses; vaccines stimulate immunity against specific pathogens.

Essential Definitions

Health

A state of physical, mental, and social well-being, enabling an individual to function optimally.

Disease

Any abnormal condition affecting the body or mind, leading to discomfort, dysfunction, or distress.

Pathogen

A disease-causing microorganism or agent, such as a virus, bacterium, fungus, or protozoan.

Vector

An organism, often an insect or tick, that transmits a disease from one host to another without itself suffering from the disease.

Infectious Disease

Diseases caused by pathogenic microorganisms, which can be spread directly or indirectly from one person to another.

Non-infectious Disease

Diseases that are not caused by pathogens and cannot be spread from one person to another; often linked to genetics, lifestyle, or environment.

Acute Disease

A disease that has a sudden onset, short duration, and often severe symptoms, like the common cold.

Chronic Disease

A disease that persists for a long time, often for life, and can have long-term effects on health, such as diabetes or tuberculosis.

Immunization

The process by which an individual's immune system becomes protected against an agent (e.g., a pathogen); often achieved through vaccination.

Antibiotics

Medicines that kill or inhibit the growth of bacteria, used to treat bacterial infections.

Understanding the Causes of Diseases

Diseases don't just happen; they have specific causes. These can be broadly categorised into immediate causes and contributory causes. Immediate causes are the primary factors directly responsible for the onset of a disease, while contributory causes are secondary factors that create conditions favorable for the immediate causes to act or worsen the disease.

Most infectious diseases are caused by pathogens, which are living organisms that invade the body. These include:

• Viruses: These are microscopic infectious agents that can only replicate inside the living cells of other organisms. Examples include the common cold, influenza, dengue fever, and AIDS. Viruses often cause diseases that are hard to treat with conventional medicines.
• Bacteria: These are single-celled microorganisms that can live in diverse environments. They cause diseases like cholera, tuberculosis, typhoid, and tetanus. Many bacterial infections can be treated with antibiotics.
• Fungi: These are eukaryotic organisms that can cause skin infections (e.g., ringworm) and some internal diseases.
• Protozoans: These are single-celled eukaryotic organisms. Diseases caused by protozoans include malaria (transmitted by _Anopheles_ mosquitoes, caused by _Plasmodium_), kala-azar (transmitted by sandflies, caused by _Leishmania_), and amoebiasis.
• Worms: These are multicellular organisms that can cause intestinal infections (e.g., ascariasis, caused by roundworms) and filariasis (elephantiasis).

Beyond pathogens, non-infectious diseases arise from various factors. These can be genetic abnormalities (e.g., haemophilia), lifestyle choices (e.g., obesity, heart disease due to poor diet and lack of exercise), environmental factors (e.g., exposure to pollutants leading to cancer or respiratory issues), or malfunctioning body organs (e.g., diabetes due to pancreatic issues). It's crucial to understand that even for infectious diseases, contributory causes like poor nutrition, lack of public hygiene, or a weak immune system can significantly increase susceptibility and severity. For example, a person exposed to cholera bacteria in contaminated water might fall ill more severely if their immune system is compromised or if there's no proper sanitation.

Infectious vs. Non-Infectious Diseases

Principles of Disease Prevention

Exam Tip: Differentiating Disease Types

In exams, students often confuse acute/chronic and infectious/non-infectious diseases. Remember: Acute diseases are _short-term_ (e.g., common cold), while Chronic diseases are _long-term_ (e.g., TB, diabetes). The key difference for Infectious diseases is their ability to _spread_ (caused by pathogens), whereas Non-infectious diseases _do not spread_ (caused by internal factors like genetics or lifestyle).

When asked about prevention, clearly distinguish between general measures (like sanitation, nutrition, hygiene, which protect against many diseases) and specific measures (like vaccination, which targets a particular disease).

Quick Revision Checks

• Q: Name two immediate causes of diseases. A: Viruses, bacteria, fungi, protozoans, or worms (any two).
• Q: What is the main difference between an acute and a chronic disease? A: Acute diseases are of short duration and typically severe, while chronic diseases are long-lasting and can have prolonged health impacts.
• Q: Give two examples of non-infectious diseases. A: Diabetes, high blood pressure, heart disease, cancer, arthritis (any two).
• Q: How do vaccines help in preventing diseases? A: Vaccines stimulate the immune system to produce antibodies against a specific pathogen without causing the actual disease, thus building immunity.

Frequently Asked Questions