NCERT Class 9 Science Chapter 13: Why Do We Fall Ill - A Complete Guide

Introduction to Why Do We Fall Ill

Welcome, students! In this comprehensive guide, we will delve into Chapter 13 of the NCERT Class 9 Science textbook, 'Why Do We Fall Ill'. This chapter is fundamental to understanding our own bodies and the world around us. It moves beyond simple definitions to explore the complex interplay between our health, our environment, and the microscopic organisms we share our planet with. We often use the terms 'health' and 'disease' interchangeably, but are they truly the same? This chapter clarifies that health is a positive and holistic state of being, encompassing physical, mental, and social well-being, while being 'disease-free' is merely the absence of discomfort. We will explore the various causes of diseases, differentiate between acute and chronic conditions, and understand the mechanisms by which illnesses spread. Most importantly, we will learn about the powerful principles of treatment and prevention, including the miracle of vaccination. A solid understanding of these concepts is not just crucial for your exams, but for leading a healthier, more informed life.

Health and its Failure

The very foundation of this chapter rests on understanding what 'health' truly means and why it sometimes fails, leading to illness. It's a concept that is deeply personal yet profoundly influenced by our community and surroundings.

The Significance of 'Health'

The World Health Organization (WHO) provides a widely accepted definition: "Health is a state of complete physical, mental and social well-being and not merely the absence of disease or infirmity." This definition is powerful because it highlights three critical dimensions of our existence:

• Physical Well-being: This is the most obvious aspect. It means your body is functioning optimally. You have the energy to perform daily tasks, your organs are working correctly, and you are free from physical pain or disability.
• Mental Well-being: This refers to your emotional and psychological state. It includes being free from anxiety and tension, having a positive outlook, and being able to cope with the normal stresses of life. A person suffering from depression or constant stress cannot be considered healthy, even if their body is physically fine.
• Social Well-being: This dimension involves our ability to interact with others and our community in a meaningful way. It means having good relationships, feeling a sense of belonging, and participating in community life. Social harmony and good public services are essential for this.

Therefore, health is a state of dynamic equilibrium. It's not just about not being sick; it's about feeling good and functioning effectively in every aspect of life.

Personal and Community Health Issues Both Matter for Health

Your personal health is not maintained in a vacuum. It is intrinsically linked to the health of your community. Consider these examples:

• Sanitation: If your neighbours do not dispose of their garbage properly, it can become a breeding ground for flies and other pests. These pests can carry disease-causing microbes and contaminate your food, making you sick. No matter how clean you keep your own home, poor community sanitation poses a risk.
• Clean Drinking Water: You might have a water purifier at home, but if the municipal water supply is contaminated with sewage, the risk of water-borne diseases like cholera and typhoid for the entire community increases. Access to safe drinking water is a community-level necessity.
• Social Environment: Living in a harmonious and supportive community contributes to mental well-being. Conversely, living in an environment with frequent conflicts or discrimination can lead to stress and anxiety, negatively impacting health.

Thus, we need organised community efforts, such as public health services, sanitation drives, and vaccination programs, to ensure the health of all individuals. Personal hygiene and healthy habits are crucial, but they must be supported by a clean and healthy community environment.

Distinctions between 'Healthy' and 'Disease-free'

This is a key concept in the chapter. While the two terms seem similar, they have distinct meanings.

• Disease-free: This is a negative concept. It simply means the absence of any specific disease. A person can be free from a diagnosable illness but still not be 'healthy'.
• Healthy: This is a positive and holistic concept. It encompasses physical, mental, and social well-being.

For example, a musician might not have any disease, but if she is unable to perform on stage due to anxiety, she is not healthy. Similarly, a dancer might be in excellent physical condition, but if he cannot express himself gracefully, his health, in the context of his profession and passion, is not optimal. Therefore, being healthy means being able to realize one's full potential.

Disease and Its Causes

When our body's functioning is disturbed, leading to discomfort, we say we have a disease. Understanding what diseases look like and what causes them is the first step toward treatment and prevention.

What does disease look like? Symptoms and Signs

When we get a disease, our body reacts. These reactions are what we call symptoms and signs.

• Symptoms: These are subjective indications of disease that we feel. They are things that tell us something is wrong, but they are not specific. Examples include a headache, cough, body ache, or feeling tired. They are clues, but they don't pinpoint the exact disease. A headache, for instance, could be due to stress, a common cold, or a more serious condition like meningitis.
• Signs: These are objective indications of disease that a doctor or another person can observe and measure. Doctors look for signs to make a more accurate diagnosis. Examples include fever (measured with a thermometer), a rash, high blood pressure, or the results of a blood test. Signs provide more definitive clues about the nature of the disease.

Acute and Chronic Diseases

Diseases can be broadly classified based on their duration.

Acute Diseases:

• They last for a very short period.
• The body usually recovers completely after the illness.
• There are no long-term bad effects on health.
• Examples: The common cold, which lasts a few days; typhoid; cholera.

Chronic Diseases:

• They last for a very long time, sometimes even for a lifetime.
• They have drastic, long-term debilitating effects on health.
• Recovery may not be complete, and the disease can cause permanent damage.
• Examples: Diabetes, tuberculosis (TB), arthritis, elephantiasis. A person with chronic TB may feel tired and lose weight over the years, affecting their ability to work and live normally.

Chronic diseases are more serious because they have a prolonged impact on a person's quality of life and general health.

Causes of Diseases

The causes of diseases are complex and can be understood at different levels.

• Immediate Causes (First-level cause): These are the primary agents that cause the disease. They are often microorganisms like viruses or bacteria. For example, the immediate cause of tuberculosis is the bacterium Mycobacterium tuberculosis.
• Contributory Causes (Second-level cause): These are factors that make a person more susceptible to the immediate cause. Why did one person get TB while others didn't, even if they were all exposed to the bacteria? The answer might lie in contributory causes. For example, a person with poor nutrition or a weak immune system is more likely to develop TB after being exposed. Lack of good nourishment is therefore a second-level cause.
• Third-level causes: These are broader societal factors. Why does a person have poor nutrition? It could be due to poverty or the unavailability of affordable, nutritious food. Thus, poverty and lack of public services can be considered third-level causes of disease.

Infectious and Non-infectious Causes

Based on their immediate causes, diseases can be grouped into two main categories.

Infectious Diseases:

• These are caused by \texternal agents, primarily microorganisms called pathogens (like viruses, bacteria, fungi, protozoa).
• They can spread from an infected person to a healthy person through various means (air, water, contact).
• They are also known as communicable diseases.
• Examples: COVID-19, influenza (flu), measles, cholera.

Non-infectious Diseases:

• These are not caused by infectious agents.
• They cannot be spread from one person to another.
• Their causes are mostly internal or related to lifestyle and genetics.
• Examples: High blood pressure (often caused by excessive weight and lack of exercise), some types of cancer (caused by genetic abnormalities or exposure to carcinogens), diabetes, and nutritional deficiencies like scurvy.

Infectious Diseases

Understanding infectious diseases is crucial for public health, as they have the potential to cause epidemics and pandemics. This section explores the agents that cause them and the ways they spread.

Infectious Agents

The world is full of living organisms, including a vast number that are too small to be seen without a microscope. The agents that cause infectious diseases fall into several categories:

• Viruses: These are tiny, non-cellular agents that can only replicate inside the living cells of other organisms. They are responsible for diseases like the common cold, influenza, dengue fever, polio, measles, and AIDS (Acquired Immuno Deficiency Syndrome), which is caused by the Human Immunodeficiency Virus (HIV).
• Bacteria: These are single-celled organisms that are found almost everywhere. While many bacteria are harmless or even beneficial, some are pathogens that cause diseases like typhoid fever, cholera, tuberculosis, anthrax, and tetanus. Antibiotics are drugs that specifically target and kill bacteria.
• Fungi: This group includes yeasts and molds. Many fungi are harmless, but some can cause diseases, especially skin infections like ringworm and athlete's foot.
• Protozoa: These are single-celled eukaryotic organisms. Pathogenic protozoa cause some of the most well-known diseases, including malaria (caused by Plasmodium, which is spread by mosquitoes) and sleeping sickness (caused by Trypanosoma).
• Worms (Helminths): Multicellular organisms like worms can also cause diseases. Intestinal worm infections are common, caused by species like tapeworms and roundworms. Another serious disease, elephantiasis (or filariasis), is caused by a species of filarial worm.

An important point to remember is that treatments must be specific to the category of the infectious agent. For instance, antibiotics work by blocking biochemical pathways important for bacteria (like cell wall synthesis). Viruses do not have these pathways, which is why antibiotics are ineffective against viral infections like the common cold.

Means of Spread

Infectious diseases are communicable, meaning they can be transmitted from a sick person to a healthy one. This transmission occurs through different routes:

• Through Air (Air-borne diseases): When an infected person coughs or sneezes, they release tiny droplets containing microbes. A healthy person breathing in this air can get infected. This is common in crowded places. Examples: Common cold, pneumonia, and tuberculosis.
• Through Water (Water-borne diseases): If the excreta from a person suffering from an infectious gut disease (like cholera) get mixed with drinking water, the microbes can enter a healthy person who drinks this water. Examples: Cholera, amoebiasis.
• Through Sexual Contact: Some diseases are transmitted through sexual contact from one partner to another. However, these diseases are not spread by casual physical contact like handshakes or hugs. Examples: Syphilis, and AIDS (HIV can also spread through blood-to-blood contact and from an infected mother to her baby during pregnancy or breastfeeding).
• Through Vectors: Many animals can carry infectious agents from a sick person to a healthy one. These animals are called vectors. The most common vectors are insects. For example, female Anopheles mosquitoes carry the protozoan that causes malaria, while Aedes mosquitoes carry the dengue virus.

Organ-specific and Tissue-specific Manifestations

Once pathogens enter our body, they travel to specific organs or tissues where they can thrive. The signs and symptoms of a disease often depend on which part of the body the microbes target.

• Point of Entry: The point of entry often determines the target organ. For example, microbes that enter through the nose via the air are likely to go to the lungs, like the bacteria causing tuberculosis. Microbes that enter through the mouth are likely to stay in the gut lining (like typhoid-causing bacteria) or go to the liver (like the virus causing jaundice).
• Target Organ and Symptoms: The symptoms we experience are a direct result of the damage to the target organ. If the lungs are the target, symptoms will include coughing and breathlessness. If the liver is the target, we see jaundice. If the brain is the target (as in Japanese encephalitis), symptoms can include headaches, vomiting, and unconsciousness.
• General Effects: In addition to organ-specific effects, the body's immune system mounts a general response to infection. This process, called inflammation, involves recruiting cells to the affected tissue to kill the pathogens. Inflammation can cause local effects like swelling and pain, and general effects like fever. In some cases, like HIV infection, the virus targets the immune system itself, weakening the body's ability to fight off other minor infections, which can then become life-threatening.

Principles of Treatment

When we fall ill with an infectious disease, there are two main approaches to treatment that are often used in combination.

1. Symptomatic Treatment (Reducing the effects of the disease)

This approach aims to reduce the symptoms, which are often due to inflammation. For instance, we can take medicines to bring down fever, reduce pain, or control loose motions. We can also take bed rest to conserve energy, allowing our body to focus on healing. This type of treatment provides relief and makes us feel better, but it does not eliminate the underlying cause—the microbes are still in the body. It essentially helps us manage the disease while our immune system does the main work of fighting it off.

2. Antimicrobial Treatment (Killing the cause of the disease)

This is the definitive approach to curing an infectious disease. It involves using medicines that kill the pathogens. As discussed earlier, we have a range of antimicrobial drugs:

• Antibiotics: For bacterial infections.
• Antivirals: For viral infections (these are harder to make than antibiotics because viruses have very few biochemical mechanisms of their own).
• Antifungals: For fungal infections.
• Antiprotozoals: For protozoan infections.

The principle behind these drugs is selective toxicity. Each group of organisms has essential biochemical life processes that are peculiar to that group. A drug is designed to block one of these specific pathways in the microbe without affecting our own cells. For example, many bacteria build a cell wall to protect themselves. Human cells do not have cell walls. The antibiotic penicillin blocks the bacterial process of cell wall synthesis, causing the bacteria to die, while our own cells remain unharmed.

Principles of Prevention

While treatment is important, there is an old saying that holds true: "Prevention is better than cure." There are three main limitations to treating an infectious disease:

1. Once someone has a disease, their body functions are damaged and may never recover completely.
2. Treatment takes time, which means the person is likely to be bedridden for a while, leading to loss of work and productivity.
3. The infected person can serve as a source from which the infection may spread to others.

Therefore, preventing the disease in the first place is the best strategy. There are two main approaches to prevention: general ways and specific ways.

General Ways of Prevention

These are methods aimed at preventing exposure to infectious agents. They are related to public hygiene and personal habits.

• For Air-borne microbes: We can prevent exposure by avoiding overcrowded places and ensuring good ventilation in our homes and schools.
• For Water-borne microbes: We can ensure the supply of safe drinking water by treating it to kill any contaminating microbes. This is a crucial public health measure.
• For Vector-borne microbes: We can provide a clean environment that does not allow for mosquito breeding. This involves clearing stagnant water from coolers, tires, and puddles.

In addition to environmental hygiene, having a strong immune system is a general way to prevent disease. The immune system is our body's defense force. If it is strong, it can often fight off and eliminate invading microbes before they cause disease. We can strengthen our immune system by eating a balanced and nutritious diet.

Specific Ways of Prevention (Immunisation)

This is a highly effective method that strengthens the immune system to fight off a specific infection. The principle of immunisation, or vaccination, is based on the 'memory' of the immune system.

When our immune system encounters a pathogen for the first time, it responds and then remembers it specifically. If the same pathogen enters the body a second time, the immune system responds with much greater vigour, eliminating the infection far more quickly than it did the first time. This is the basis of immunity.

Vaccination 'fools' the immune system. A vaccine contains a harmless version or a part of the pathogen (weakened or killed) that cannot cause the disease but is sufficient to be recognized as foreign by the immune system. When this is introduced into the body, the immune system produces antibodies and memory cells against it. Later, when the actual pathogen enters the body, the memory cells recognize it instantly and mount a massive and rapid attack, preventing the disease from developing. This provides specific immunity against a particular disease.

We now have vaccines against a host of diseases, including tetanus, diphtheria, whooping cough, measles, polio, and many others, which have saved millions of lives worldwide.

Important Questions and Answers

Q1: Differentiate between acute and chronic diseases.

Answer: The differences between acute and chronic diseases can be summarized in the following table:

Q2: Why are we normally advised to take bland and nourishing food when we are sick?

Answer: When we are sick, our body's normal functions are disrupted, and our immune system is working overtime to fight off the infection. We are advised to take bland and nourishing food for two main reasons:

1. Easy Digestion (Bland Food): The digestive system may not be functioning at its peak during illness. Spicy, oily, or heavy foods are difficult to digest and can put \textra strain on the body, potentially worsening symptoms like nausea or stomach upset. Bland food, such as boiled vegetables, porridge, or soup, is easy to digest, allowing the body to conserve energy. This conserved energy can then be redirected towards the immune system to fight the disease-causing microbes.
2. Energy and Nutrients (Nourishing Food): The immune system requires a significant amount of energy and specific nutrients (like vitamins and minerals) to function effectively and produce immune cells and antibodies. Nourishing food provides the essential raw materials and energy needed for the body to heal and recover from the illness. A good diet strengthens the immune response, helping to clear the infection more quickly.

Q3: What are the different means by which infectious diseases are spread?

Answer: Infectious diseases, or communicable diseases, are spread from an infected person to a healthy person through various means involving different agents. The primary means of spread are:

• Through Air (Air-borne): Microbes are transmitted through tiny droplets released when an infected person coughs, sneezes, or talks. A healthy person inhaling this contaminated air can get infected. Examples: Common cold, pneumonia, tuberculosis.
• Through Water (Water-borne): Pathogens from the excreta of an infected person can contaminate drinking water sources. Consuming this contaminated water leads to infection. Examples: Cholera, amoebic dysentery, typhoid.
• Through Direct Contact: Some diseases spread through physical contact with an infected person. This can be through skin-to-skin contact or sexual contact. Examples: Skin infections like ringworm, and sexually transmitted diseases (STDs) like syphilis and AIDS.
• Through Indirect Contact (Fomites): Microbes can be transferred via inanimate objects used by an infected person, such as clothes, utensils, or toys. A healthy person touching these objects and then their face can get infected.
• Through Vectors: These are animals that carry pathogens from an infected individual to a healthy one without getting sick themselves. Mosquitoes are common vectors, spreading diseases like malaria (Anopheles mosquito) and dengue (Aedes mosquito). Flies are another common vector.

Q4: What precautions can you take in your school to reduce the incidence of infectious diseases?

Answer: To reduce the incidence of infectious diseases in school, we can take several precautions based on the principles of prevention. These can be categorized into personal and community (school-level) measures:

• Ensuring Cleanliness: The school premises, including classrooms, toilets, and playgrounds, should be cleaned and disinfected regularly.
• Providing Safe Drinking Water: The school must ensure that the water available for drinking is safe and purified to prevent water-borne diseases.
• Promoting Personal Hygiene: Students should be encouraged to wash their hands with soap and water frequently, especially before meals and after using the toilet.
• Preventing Overcrowding: Classrooms should be well-ventilated and not overcrowded to reduce the spread of air-borne diseases.
• Waste Management: Proper disposal of waste in covered bins is essential to prevent the breeding of flies and other vectors.
• Staying Home When Sick: Students and staff who are sick with a communicable disease should be encouraged to stay home to avoid spreading the infection to others.
• Health Education: Conducting awareness programs about hygiene, nutrition, and vaccination to educate students about how diseases spread and how they can be prevented.
• Preventing Vector Breeding: Ensuring there is no stagnant water in or around the school (in coolers, pots, pits) to prevent mosquitoes from breeding.

Chapter Summary

Here are the key takeaways from the chapter 'Why Do We Fall Ill':

• Health is a state of complete physical, mental, and social well-being, not just the absence of disease.
• Both personal health and community health are interconnected and essential for individual well-being.
• Diseases can be acute (short-term) or chronic (long-term). Chronic diseases have a more debilitating effect on health.
• The causes of disease can be infectious (caused by microbes and are communicable) or non-infectious (caused by genetics, lifestyle, etc., and are non-communicable).
• Infectious agents include viruses, bacteria, fungi, protozoa, and worms.
• Infectious diseases spread through various means: air, water, vectors, and physical contact.
• The signs and symptoms of a disease depend on the organ or tissue targeted by the microbes.
• The principles of treatment involve either reducing the symptoms (symptomatic) or killing the microbes (antimicrobial).
• Prevention is better than cure. Prevention strategies can be general (maintaining hygiene, healthy diet) or specific.
• Immunisation (vaccination) is a specific method of prevention that uses the 'memory' of the immune system to protect against a particular disease.