Why Do We Fall Ill?

Why Do We Fall Ill? - Class 9 Science

  • Health
  • Distinction Between Healthy and Disease-free
  • Principles of Disease Treatment
  • Principles of Prevention
  • Specific Disease Prevention through Immunization
  • Solved Questions on Why Do We Fall Ill?
  • Health

    Health refers to a state of complete physical, mental, and social well-being, not merely the absence of disease. It encompasses various dimensions of well-being that contribute to an individual's overall quality of life. Good health is vital for leading a productive and fulfilling life.

    Factors Affecting Health

    1. Physical Environment: The surroundings in which an individual lives, including air quality, water supply, sanitation, and living conditions, significantly impact health.
    2. Social Environment: The quality of relationships, social support systems, and community cohesion influence mental and emotional well-being.
    3. Socioeconomic Status: Economic factors, such as income, education, and access to healthcare, determine an individual's ability to maintain good health.
    4. Access to Healthcare: Availability of medical facilities, quality healthcare services, and timely medical attention play a crucial role in maintaining health.
    5. Genetics: Genetic predisposition can influence susceptibility to certain diseases or conditions.
    6. Lifestyle Choices: Habits like diet, physical activity, sleep patterns, and substance use directly impact health outcomes.
    7. Mental Health: Emotional well-being, stress management, and psychological resilience contribute to overall health.

    Failure of Health

    1. Health failure occurs when an individual's physical, mental, or social well-being deteriorates, leading to various negative outcomes. This failure can manifest in different ways such as physical, mental, social, economic, and environmental.
    2. Physical health failure results from diseases, injuries, or chronic conditions, leading to symptoms like pain and weakness.
    3. Mental health failure involves psychological decline, often due to conditions like depression and anxiety.
    4. Economic health failure relates to financial struggles hindering access to healthcare and nutritious food.
    5. Environmental health failure stems from exposure to hazards, and healthcare system failure results from inadequate access to quality medical care and resources.

    Personal and Community Issues: Both Matter for Health

    Personal Factors Affecting Health

    1. Lifestyle Choices: Personal choices like diet, physical activity, smoking, alcohol consumption, and substance use significantly impact health. A healthy lifestyle reduces the risk of chronic diseases and promotes overall well-being.
    2. Genetic Predisposition: Genetic factors can influence an individual's susceptibility to certain diseases. While genetics play a role, lifestyle modifications can still mitigate the risk.
    3. Mental Health: Personal mental health, including managing stress, anxiety, and depression, contributes to overall health. Positive mental health promotes resilience and coping mechanisms.
    4. Hygiene and Self-Care: Personal hygiene practices, such as regular handwashing, dental care, and cleanliness, play a vital role in preventing infections and maintaining health.

    Community Factors Affecting Health

    1. Access to Healthcare: Communities with easily accessible and quality healthcare facilities ensure that individuals receive timely medical attention and necessary treatments.
    2. Public Health Infrastructure: Availability of clean water, sanitation, waste management, and vaccination programs in a community directly impact disease prevention and health promotion.
    3. Social Support Networks: Strong community bonds and social support systems provide individuals with emotional well-being and a safety net during times of illness or stress.
    4. Environmental Quality: Communities with clean air, safe housing, green spaces, and reduced pollution levels contribute to better physical and mental health.
    5. Education and Awareness: Communities that prioritise health education and awareness campaigns empower individuals to make informed decisions about their health and adopt healthier lifestyles.
    6. Employment Opportunities: Access to stable employment and economic opportunities ensures that individuals can afford healthcare, nutritious food, and a healthy lifestyle.

    Distinction Between Healthy and Disease-free

    The distinction between being "healthy" and "disease-free" lies in the broader context of overall well-being and the absence of illness. While the terms might seem similar, they address different aspects of an individual's state:


    1. Being "healthy" implies a state of complete physical, mental, and social well-being. It goes beyond the mere absence of disease. A healthy person not only lacks illness but also enjoys vitality, optimal bodily functions, mental clarity, emotional balance, and the ability to engage actively in various aspects of life.
    2. Good health includes a balanced diet, regular exercise, mental wellness, strong social connections, and a positive outlook on life. It's a holistic state of being that promotes a high quality of life.


    1. Being "disease-free" simply indicates the absence of a specific illness or disease at a given point in time. It doesn't necessarily guarantee overall well-being or good health.
    2. An individual can be disease-free but still experience other health issues, such as poor mental health, lack of physical fitness, or social isolation. In this context, the focus is on the absence of a particular ailment without addressing the individual's broader health status.


    Disease is a broad term used to describe any condition or disorder that disturbs the normal functioning of the body, leading to physical, mental, or emotional discomfort. Diseases can range from mild and temporary to severe and chronic, affecting individuals' well-being and quality of life.

    Types of Diseases

    1. Infectious Diseases: These are caused by microorganisms, such as bacteria, viruses, fungi, and parasites. Examples include the common cold, tuberculosis, HIV/AIDS, and malaria.
    2. Non-Infectious Diseases: These are not caused by microorganisms and include a wide range of conditions like cardiovascular diseases (heart diseases), diabetes, cancer, and autoimmune disorders.
    3. Genetic Disorders: These are caused by abnormalities in an individual's DNA. They can be inherited from parents and may manifest at birth or later in life.
    4. Degenerative Diseases: These involve the progressive deterioration of tissues and organs over time, often associated with ageing. Examples include Alzheimer's disease and osteoarthritis.
    5. Mental Health Disorders: These affect mental and emotional well-being, leading to disturbances in mood, behaviour, and cognition. Examples include depression, anxiety disorders, and schizophrenia.

    Classification of Diseases based on Duration

    Diseases can be categorised based on their duration and progression. Acute and chronic diseases are two primary classifications that help describe how long a disease lasts and how it progresses within the body.

    a) Acute Diseases

    Acute diseases are categorised by their sudden onset and relatively short duration. They typically develop rapidly, often within hours or a few days. Acute diseases tend to have intense and severe symptoms that can significantly impact an individual's health and well-being. Here are some key features of acute diseases:

    1. Rapid Onset: Acute diseases develop suddenly, with symptoms appearing quickly after exposure to the disease-causing agent.
    2. Short Duration: Acute diseases have a relatively short course, usually lasting for a few days to a few weeks.
    3. Intense Symptoms: The symptoms of acute diseases are often severe and pronounced. They may include high fever, severe pain, nausea, vomiting, and rapid deterioration of health.
    4. Resolution: Many acute diseases resolve on their own or with medical intervention. Once the body's immune system effectively fights off the infection or the underlying cause is addressed, the symptoms subside, and the person recovers.
    5. Examples: Common cold, influenza, strep throat, gastroenteritis, and appendicitis are examples of acute diseases.

    b) Chronic Diseases

    Chronic diseases, on the other hand, have a prolonged and persistent course. They develop gradually over time and often last for months, years, or even a lifetime. Chronic diseases may start with mild symptoms that worsen gradually or periodically. Some key features of chronic diseases are:

    1. Gradual Onset: Chronic diseases develop gradually, and symptoms may take weeks, months, or even years to become noticeable.
    2. Long Duration: Chronic diseases persist over an extended period, often for years. They can have a significant impact on an individual's quality of life.
    3. Intermittent Symptoms: Symptoms of chronic diseases may come and go, with periods of exacerbation and remission. These fluctuations can make management challenging.
    4. Less Severe Initial Symptoms: The initial symptoms of chronic diseases are usually milder compared to acute diseases. Over time, these symptoms can worsen and lead to complications.
    5. Management: Chronic diseases often require ongoing medical care, management, and lifestyle adjustments to control symptoms, slow progression, and prevent complications.
    6. Examples: Diabetes, hypertension, asthma, chronic obstructive pulmonary disease (COPD), arthritis, and cardiovascular diseases are examples of chronic diseases.

    Causes of Diseases

    Diseases can have various underlying causes that contribute to their development. These causes can be broadly categorised into infectious and non-infectious factors. Understanding the different causes of diseases is essential for effective prevention, treatment, and management.

    a) Infectious Causes of Diseases

    Infectious causes of diseases are attributed to the presence and activity of pathogens. Pathogens are microorganisms or agents that can cause diseases in their hosts. These disease-causing agents can be viruses, bacteria, fungi, protozoa, and even multicellular parasites like worms. Pathogens have specific properties and mechanisms that allow them to invade, colonise, and disrupt the normal functioning of the host's body, leading to various health issues.

    b) Non-Infectious Causes of Diseases

    Non-infectious causes of diseases are not related to microorganisms. These causes stem from internal factors, genetic predisposition, lifestyle choices, environmental factors, and more. Some non-infectious causes of diseases are:

    1. Genetic Abnormalities: Genetic mutations or abnormalities inherited from parents can predispose individuals to certain diseases. Examples include cystic fibrosis, sickle cell anaemia, and haemophilia.
    2. Lifestyle Factors: Unhealthy lifestyle choices, such as poor diet, lack of physical activity, smoking, excessive alcohol consumption, and drug abuse, can contribute to the development of diseases like obesity, heart disease, and type 2 diabetes.
    3. Environmental Factors: Exposure to environmental pollutants, toxins, radiation, and harmful chemicals can increase the risk of diseases such as lung cancer (due to air pollution), skin cancer (due to UV radiation), and occupational lung diseases (due to workplace exposures).
    4. Hormonal Imbalances: Imbalances in hormonal levels can lead to various disorders, including thyroid disorders, diabetes, and polycystic ovary syndrome (PCOS).
    5. Ageing: Ageing is a natural process that can weaken the body's immune system and increase susceptibility to chronic diseases such as arthritis, osteoporosis, and age-related cognitive decline.
    6. Nutritional Deficiencies: Inadequate intake of essential nutrients can lead to deficiencies and related diseases, such as iron deficiency anaemia and vitamin D deficiency disorders.

    Types of Infectious Agents and Their Diseases

    Infectious diseases are caused by a variety of pathogens, each belonging to different categories based on their biological characteristics. Understanding these categories and the diseases they cause is crucial for diagnosing, treating, and preventing infections. Some of the main types of infectious agents and their associated diseases are:


    Biological Characteristics

    Disease Caused


    Bacteria are single-celled microorganisms that can exist in various shapes and sizes.

    Tuberculosis (Mycobacterium tuberculosis)

    Strep throat (Streptococcus pyogenes)

    Salmonellosis (Salmonella spp.)

    Cholera (Vibrio cholerae)

    Urinary tract infections (Escherichia coli)

    Bacterial pneumonia (Streptococcus pneumoniae)


    Protozoa are single-celled eukaryotic organisms that can cause diseases.

    Malaria (Plasmodium spp.)

    Amoebic dysentery (Entamoeba histolytica)

    Sleeping sickness (Trypanosoma spp.)

    Toxoplasmosis (Toxoplasma gondii)


    Viruses are small infectious agents that can only replicate inside host cells.

    Influenza (Influenza virus)

    Common cold (Rhinovirus)

    Human immunodeficiency virus (HIV)

    Hepatitis (Hepatitis B and C viruses)

    COVID-19 (Severe acute respiratory syndrome coronavirus 2, or SARS-CoV-2)

    Helminths (Parasitic Worms)

    Helminths are multicellular parasites that can infect humans and animals.

    Intestinal worm infections (Ascaris, hookworm, whipworm)

    Schistosomiasis (Schistosoma spp.)

    Elephantiasis (Filariasis)

    Means of Disease Spread

    Infectious diseases can spread from one person to another through various means, leading to the transmission of infectious agents. Understanding how these diseases spread is crucial for implementing effective preventive measures. Some of the main means through which diseases can spread are:

    Type of Transmission



    Airborne Transmission

    Microorganisms can be present in tiny respiratory droplets released when an infected person coughs, sneezes, talks, or breathes.

    Others can inhale these droplets, leading to the spread of diseases.

    Common cold




    Waterborne Transmission

    Contaminated water sources can carry infectious agents, allowing them to enter the body through ingestion or contact with open wounds.


    Hepatitis A

    Typhoid fever

    Foodborne Transmission

    Consuming contaminated food, often due to improper handling, preparation, or storage, can introduce infectious agents into the body.


    E. coli infections

    Hepatitis A

    Direct Contact Transmission

    Physical contact with an infected person or their bodily fluids can lead to disease transmission.

    Sexually transmitted infections (STIs) like HIV, syphilis, and herpes

    Skin infections like impetigo and scabies

    Indirect Contact Transmission

    Indirect contact involves touching contaminated surfaces or objects and then touching the face or mouth, introducing pathogens into the body.

    Norovirus infections

    Respiratory infections like the common cold

    Vector-Borne Transmission

    Vectors are organisms that can carry infectious agents from one host to another. They can be insects like mosquitoes or ticks.

    Malaria (mosquitoes)

    Lyme disease (ticks)

    Dengue fever (mosquitoes)

    Zika virus (mosquitoes)

    Vertical Transmission

    Some infectious agents can be transmitted from a pregnant woman to her fetus or newborn during childbirth or breastfeeding.



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    Principles of Disease Treatment

    Principles of disease treatment encompass the strategies and approaches used to alleviate the symptoms, eliminate the causative agents, and promote recovery in individuals who are suffering from various illnesses. The goal of disease treatment is to restore the individual's health and well-being. Some of the key principles of disease treatment are:

    1. Symptom Relief

    1. Treatment often begins with addressing the symptoms that a patient is experiencing. This can involve providing medications or interventions to alleviate pain, reduce fever, ease discomfort, and improve overall well-being.
    2. Symptom relief aims to enhance the patient's quality of life while the underlying cause of the disease is being addressed.

    2. Elimination of Causative Agents

    1. For infectious diseases caused by microorganisms like bacteria, viruses, fungi, and parasites, treatment involves targeting and eliminating the causative agents. This can be achieved through the use of specific medications.
    2. Antibiotics are powerful drugs that target and kill bacteria or inhibit their growth. Similarly, antiviral, antifungal, and antiparasitic agents are used to treat diseases caused by viruses, fungi, and parasites, respectively. These medications are designed to disrupt the specific biological processes of these microorganisms, minimising their impact on the body.

    3. Specificity

    Effective treatment often requires identifying the precise cause of the disease. Different diseases have distinct causative agents, and treatment needs to be tailored to target those agents. This principle underscores the importance of accurate diagnosis.

    4. Duration of Treatment

    The duration of treatment varies based on the type and severity of the disease. It is crucial to complete the full course of prescribed treatment to ensure that all causative agents are eliminated and to prevent the development of drug-resistant strains.

    5. Combination Therapy

    In some cases, a combination of medications may be used to target multiple aspects of the disease or to prevent the development of resistance. For example, highly active antiretroviral therapy (HAART) is used to manage HIV/AIDS by combining different antiviral drugs.

    6. Supportive Care

    Providing supportive care, such as fluids, nutrition, rest, and pain relief, is essential to help the body recover and cope with the effects of the disease and its treatment. Supportive care may also include interventions like physiotherapy or counselling.

    7. Individualisation

    Treatment plans should be tailored to the individual patient's needs, taking into account factors such as age, overall health, medical history, and potential interactions with other medications the patient may be taking.

    8. Preventing Complications

    Effective treatment aims to prevent complications that may arise due to the disease. Timely and appropriate intervention can reduce the risk of further health issues.

    9. Patient Education

    Educating patients about their disease, treatment options, potential side effects of medications, and lifestyle changes is important for promoting compliance with treatment plans and enabling patients to actively participate in their recovery.

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    Principles of Prevention

    1. Preventing diseases is a crucial aspect of maintaining good health. The most effective way to ensure good health is to take measures that prevent diseases from occurring in the first place. Preventive strategies reduce the risk of diseases and their associated complications.
    2. Preventing diseases is more advantageous than treating them later.
    3. Treatment may lead to prolonged damage to bodily functions, extended bed rest, and potential transmission of the disease to others.
    4. There are various principles and methods of disease prevention that can be classified into general and specific approaches.

    1. Public Health Hygiene

    1. Maintaining clean and hygienic living environments helps prevent the spread of infectious agents.
    2. Practices such as proper waste disposal, clean water supply, and sanitation contribute to preventing waterborne and vector-borne diseases.

    2. Vaccination and Immunization

    1. Vaccines introduce weakened or inactive forms of pathogens into the body, stimulating the immune system to develop immunity without causing disease.
    2. Immunisation programs target specific diseases and provide long-term protection against them.

    3. Personal Hygiene

    1. Practising good personal hygiene, such as regular handwashing, can prevent the transmission of infectious agents.
    2. Covering the mouth and nose when sneezing or coughing reduces the spread of respiratory infections.

    4. Healthy Lifestyle

    Adopting a healthy lifestyle through balanced nutrition, regular physical activity, adequate sleep, and stress management boosts the immune system and overall health.

    5. Vector Control

    1. Controlling vectors such as mosquitoes and ticks helps prevent the transmission of diseases like malaria, dengue fever, and Lyme disease.
    2. Measures may include using insect repellents, eliminating breeding sites, and using protective clothing.

    6. Safe Food Practices

    Proper food handling, storage, and cooking can prevent foodborne illnesses caused by contaminated food.

    7. Screening and Early Detection

    1. Regular health check-ups and screenings help detect diseases in their early stages when they are more treatable.
    2. Early detection and prompt intervention can prevent disease progression.

    8. Travel Health Precautions

    Taking necessary health precautions before travelling, such as getting required vaccines and taking preventive medications, helps prevent diseases prevalent in certain regions.

    9. Health Policies and Regulations

    1. Government policies and regulations play a vital role in disease prevention, such as mandatory vaccinations and regulations on food safety.
    2. Ensuring everyone has access to healthcare services, regardless of socioeconomic status, improves overall health and prevents disparities in disease outcomes.

    Specific Disease Prevention through Immunization

    Immunisation is a powerful and effective approach to prevent specific infectious diseases. It involves introducing the body to a harmless form of a pathogen, often a weakened or inactivated version, to stimulate the immune system's response. This process results in the development of immunity so that if the individual encounters the actual disease-causing pathogen in the future, their immune system can quickly recognize and fight it off. Some of the important aspects of immunisation are:

    1. Introduction of Weakened or Inactivated Pathogens

    1. Vaccines contain components of disease-causing microorganisms, such as weakened viruses, inactivated bacteria, or parts of the pathogen (like proteins).
    2. These components cannot cause the disease itself but are enough to trigger the immune response.

    2. Stimulating Immune Response

    1. When a vaccine is administered, the immune system recognizes the foreign components and generates an immune response.
    2. Immune cells are activated to produce antibodies and immune memory cells.

    3. Formation of Immune Memory

    1. After the immune response, the body retains memory cells specific to the pathogen's antigens.
    2. If the person encounters the actual pathogen later, the immune system can rapidly mount a stronger and quicker response, preventing disease development.

    4. Herd Immunity (Community Immunity)

    1. When a significant portion of a population is immune to a disease, either through vaccination or previous infection, the spread of the disease is limited.
    2. Herd immunity protects vulnerable individuals who cannot be vaccinated, such as those with weakened immune systems or allergies.

    5. Types of Vaccines

    1. Live Attenuated Vaccines: These contain weakened live pathogens that replicate at a slower rate and cause a mild or asymptomatic infection. Examples include the measles, mumps, and rubella (MMR) vaccine.
    2. Inactivated Vaccines: These vaccines contain killed pathogens or parts of the pathogen. They stimulate the immune system without causing disease. Examples include the hepatitis B vaccine.
    3. Subunit, Recombinant, or Conjugate Vaccines: These vaccines use specific components of the pathogen to trigger an immune response. They are safer for individuals with weakened immune systems. Examples include the Haemophilus influenzae type b (Hib) vaccine.
    4. mRNA Vaccines: A newer technology that introduces genetic material (mRNA) encoding a pathogen's antigen. The body's cells produce the antigen, triggering an immune response. The COVID-19 vaccines are mRNA vaccines.

    6. Vaccine Schedule

    1. Most vaccines require multiple doses to establish strong and long-lasting immunity.
    2. A vaccination schedule outlines when each dose should be administered for optimal protection.

    7. Public Health Impact

    1. Immunisation programs have played a significant role in reducing the incidence of many deadly diseases.
    2. Diseases like smallpox and polio have been eradicated or significantly reduced due to widespread vaccination efforts.

    8. Importance of Equitable Access

    Ensuring that all individuals, regardless of socioeconomic status or geographical location, have access to vaccines is crucial for global disease prevention efforts.

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