Table of Content
  1. Introduction to health
  2. Common Diseases in Humans
  3. Immunity
    1. Innate Immunity
    2. Acquired Immunity
    3. Active and Passive Immunity
    4. Vaccination and Immunization
    5. Allergies
    6. Auto Immunity
    7. Immune System in the Body
  4. AIDS
  5. Cancer
  6. Drugs and Alcohol Abuse
    1. Adolescence and Drug/Alcohol Abuse
    2. Addiction and Dependence
    3. Effects of Drug/Alcohol Abuse
    4. Prevention and Control

The Definition of Health

“Health is not just the absence of disease, it is a state of complete physical, mental, and social well-being.”

World Health Organization

Health is a state of complete physical, mental, and social well-being. It’s important to have a balanced diet, personal hygiene, and regular exercise to maintain good health.

Awareness about diseases and their effects on different bodily functions, proper vaccination, hygienic food and water, and waste management are essential to achieve good health.

The Evolution of Health

For a long time, health was considered as a balance of certain “humors.” Early Greeks and the Indian Ayurveda system of medicine believed in this idea.

However, the discovery of blood circulation by William Harvey and the use of thermometer to measure normal body temperature disproved the “good humor” hypothesis of health. Later, it was discovered that the mind influences our immune system and health through the neural and endocrine systems.

Factors Affecting Health:

  • Genetic disorders
  • Infections
  • Lifestyle (diet, exercise, habits, etc.)

Importance of Good Health:

  • Productivity and economic prosperity
  • Longevity
  • Reduced infant and maternal mortality

Maintaining Good Health:

Balanced diet, personal hygiene, and regular exercise are crucial to maintain good health.

Yoga has been practiced for centuries to achieve physical and mental health. Proper waste management, hygienic food and water resources, and vaccination are necessary to achieve good health.


Diseases can be broadly categorized into infectious and non-infectious diseases.

  • Infectious diseases are easily transmitted from one person to another,
  • Non-infectious diseases, such as cancer, are major causes of death.

Drug and alcohol abuse also affect our health adversely.

Common Diseases in Humans

A wide range of organisms including bacteria, viruses, fungi, protozoans, and helminths can cause diseases in humans. These disease-causing organisms are called pathogens and most parasites are pathogens as they cause harm to the host by living in or on them.

Pathogens enter our body through various means, multiply, and interfere with normal vital activities, leading to morphological and functional damage. The pathogens have to adapt to life within the host’s environment. Preventive and control measures against these diseases are briefly described.

Table of Disease, Causing Agent, Mode, Symptoms and effects

DiseaseCausing Agent/PathogenVector/Mode of InfectionSymptomsEffects
Typhoid feverSalmonella typhiContaminated food and waterSustained high fever (39° to 40°C), weakness, stomach pain, constipation, headache, loss of appetiteIntestinal perforation and death in severe cases
PneumoniaStreptococcus pneumoniae, Haemophilus influenzaeInhaling droplets/aerosols from infected person, sharing glasses and utensilsFever, chills, cough, headache. In severe cases, lips and finger nails may turn gray to bluish in colorSevere problems in respiration
Common coldRhino virusesInhaled droplets, contaminated objectsNasal congestion and discharge, sore throat, hoarseness, cough, headache, tirednessNasal congestion and discharge, sore throat, hoarseness, cough, headache, tiredness (usually last for 3-7 days
MalariaPlasmodium (P. vivax, P. malaria, P. falciparum)Bite of infected female Anopheles mosquitoChill and high fever recurring every 3-4 daysMalignant malaria caused by P. falciparum can be fatal
Amoebiasis (amoebic dysentery)Entamoeba histolyticaHouseflies, contaminated drinking water and foodConstipation, abdominal pain, cramps, stools with excess mucous and blood clotsContaminates food and water
AscariasisAscarisContaminated water, vegetables, fruits, etc.Internal bleeding, muscular pain, fever, anemia, blockage of the intestinal passageContaminates soil, water, plants, etc.
Elephantiasis (Filariasis)Wuchereria (W. bancrofti and W. malayi)Bite by female mosquito vectorsSlowly developing chronic inflammation of lymphatic vessels in lower limbs and genital organs, resulting in gross deformitiesSlowly developing chronic inflammation of the lymphatic vessels of the lower limbs and genital organs, resulting in gross deformities
RingwormsMicrosporum, Trichophyton, EpidermophytonSoil or using towels, clothes, or comb of infected individualsDry, scaly lesions on various parts of the body such as skin, nails, and scalp accompanied by intense itchingThrives in heat and moisture

Life Cycle of Plasmodium (Malarial Parasite)

The lifecycle of Plasmodium (malarial parasite) can be described as follows:

  1. A female Anopheles mosquito bites an infected human and ingests Plasmodium-infected blood.
  2. The sporozoites from the infected red blood cells enter the mosquito’s salivary glands.
  3. The mosquito then bites another human and transmits the sporozoites through its saliva.
  4. The sporozoites travel to the liver and infect liver cells, where they multiply and form merozoites.
  5. The merozoites then enter the red blood cells and multiply again, causing the infected red blood cells to rupture and release more haemozoin into the bloodstream. [Q. Explain the cause of periodic recurrence of chill and high fever during the malarial attack in humans.]
  6. The cycle of red blood cell infection and rupture continues, leading to the symptoms of malaria.


This overall ability of the host to fight the disease-causing organisms, conferred by the immune system is called immunity.

Immunity is of two types:

  • (i) Innate immunity
  • (ii) Acquired immunity

Innate Immunity: The Body’s First Line of Defense

Innate immunity is the body’s first line of defense against pathogens and disease-causing agents. It is the body’s non-specific response to foreign substances and is activated immediately upon exposure.

This type of immunity is present at birth and is composed of physical and chemical barriers, such as the skin, mucous membranes, and enzymes, which prevent pathogens from entering the body. Additionally, white blood cells, also known as phagocytes, are responsible for engulfing and destroying foreign substances that manage to penetrate the physical barriers.

Types of Barriers

Humans have innate immunity for protection against pathogens that may enter the gut along with food, there are different types of barriers for protection.

Physical barrier

Skin on our body is the main barrier which prevents entry of the micro-organisms. Mucus coating of the epithelium lining the respiratory, gastrointestinal and urogenital tracts also help in trapping microbes entering our body.

Physiological barriers

Acid in the stomach, saliva in the mouth, tears from eyes–all prevent microbial growth.

Cellular barriers

Certain types of leukocytes (WBC) of our body like polymorpho-nuclear leukocytes (PMNL-neutrophils) and monocytes and natural killer (type of lymphocytes) in the blood as well as macrophages in tissues can phagocytose and destroy microbes.

Cytokine barriers

Virus-infected cells secrete proteins called interferons which protect non-infected cells from further viral infection.

Adaptive Immunity: The Body’s Second Line of Defense

Acquired immunity, also known as Adaptive immunity, is the body’s second line of defense against pathogens. It is a more specific response to foreign substances and takes longer to develop than innate immunity.

Adaptive immunity is made up of B cells, which produce antibodies, and T cells, which directly target and destroy infected cells. This type of immunity is developed over time through exposure to foreign substances, such as vaccinations and previous infections, and allows the body to quickly recognize and respond to subsequent exposures.

  • Is pathogen specific and characterized by memory.
  • Body produces a low-intensity primary response upon first encounter with a pathogen, followed by a highly intensified secondary response upon subsequent encounters.
  • Primary and secondary immune responses are carried out with the help of B-lymphocytes and T-lymphocytes.
    • B-lymphocytes produce antibodies to fight pathogens.
    • T-lymphocytes help B-lymphocytes produce antibodies but do not secrete them.
  • Different types of antibodies (IgA, IgM, IgE, IgG) are produced in the body, leading to a response called the humoral immune response or antibody-mediated immune response.
    • IgA: This antibody is found in body secretions, such as saliva, tears, and breast milk, and helps protect against infections in mucous membranes.
    • IgM: This is the first antibody to appear in response to an initial infection and is the largest and most complex antibody.
    • IgE: This antibody is associated with allergic reactions and helps to protect against parasites.
    • IgG: This is the most abundant antibody in the body and is involved in both short-term and long-term protection against infections.
  • The second type of acquired immune response is cell-mediated immunity, mediated by T-lymphocytes.
  • Body can differentiate “self” and “non-self” and cell-mediated immunity is responsible for graft rejection.

Active and Passive Immunity

When a host is exposed to antigens, which may be in the form of living or dead microbes or other proteins, antibodies are produced in the host body. This type of immunity is called active immunity.

When ready-made antibodies are directly given to protect the body against foreign agents, it is called passive immunity.

  • Active immunity takes time to give full effect and can be induced through immunization or natural infection.
  • Passive immunity occurs when ready-made antibodies are directly given to protect the body against foreign agents.
  • Mother’s milk is essential for newborns due to its colostrum (The yellowish fluid secreted by mother during the initial days of lactation ), which has abundant antibodies (IgA) to protect the infant. [Q. Why is breast-feeding recommended during the initial period of an infant’s growth]
  • Foetus receives some antibodies from the mother through the placenta during pregnancy, another example of passive immunity.

Vaccination and Immunisation

  • Immunization/vaccination is based on the immune system’s property of memory.
  • In vaccination, antigenic proteins of pathogens or inactivated/weakened pathogens are introduced into the body.
  • The antibodies produced against these antigens neutralize pathogenic agents during actual infection.
  • Vaccines also generate memory B and T-cells that recognize the pathogen quickly upon subsequent exposure and produce a massive amount of antibodies.
  • Passive immunization occurs when preformed antibodies or antitoxins are directly injected in the case of quick immune response required diseases (e.g. tetanus) or snakebites.
  • Recombinant DNA technology allows the production of antigenic polypeptides of pathogens in bacteria or yeast, leading to large-scale vaccine production (e.g. hepatitis B vaccine).
  • The goal of immunization is to provide immunity to the individual against the target disease without having to experience the disease itself.


  • Allergy is an exaggerated response of the immune system to certain antigens in the environment.
  • Allergens are substances that trigger an immune response in the body.
  • The antibodies produced in response to allergens are of the IgE type.
  • Common allergens include mites in dust, pollens, animal dander, etc.
  • Symptoms of allergic reactions include sneezing, watery eyes, running nose, and difficulty in breathing.
  • Allergy is caused by the release of chemicals like histamine and serotonin from mast cells.
  • The cause of allergy can be determined by exposing the patient to or injecting them with very small doses of possible allergens.
  • Drugs such as anti-histamines, adrenaline, and steroids can quickly reduce allergy symptoms.
  • Modern-day lifestyles have resulted in a lowering of immunity and increased sensitivity to allergens.
  • More and more children in metro cities of India suffer from allergies and asthma due to sensitivity to the environment, potentially due to a protected environment provided early in life.

Auto Immunity

  • Memory-based acquired immunity evolved in higher vertebrates
  • Ability to differentiate foreign organisms (pathogens) from self-cells
  • Higher vertebrates can distinguish foreign molecules and organisms
  • Most experimental immunology deals with foreign molecule differentiation
  • Sometimes, body attacks self-cells resulting in auto-immune diseases
  • Examples of auto-immune diseases: Rheumatoid arthritis

Immune System in the Body

  • The human immune system consists of lymphoid organs, tissues, cells and soluble molecules like antibodies.
  • It recognizes and responds to foreign antigens, and also plays a role in allergic reactions, auto-immune diseases and organ transplantation.
  • Lymphoid organs include the primary organs (bone marrow and thymus) where immature lymphocytes differentiate into antigen-sensitive cells, and the secondary organs (spleen, lymph nodes, tonsils, Peyer’s patches, and appendix) where the interaction of lymphocytes with antigens occur.
  • The bone marrow is the main lymphoid organ for producing all blood cells including lymphocytes.
  • The thymus is large at birth but reduces in size with age.
  • The spleen acts as a filter for blood and contains a large reservoir of erythrocytes.
  • Lymph nodes are small structures located along the lymphatic system that trap micro-organisms and activate lymphocytes.
  • There is also lymphoid tissue located within the lining of major tracts (respiratory, digestive, and urogenital) called mucosa-associated lymphoid tissue (MALT), which constitutes about 50% of the lymphoid tissue in the human body.

AIDS  (Acquired Immuno Deficiency Syndrome)

  • AIDS stands for Acquired Immuno Deficiency Syndrome, which is a deficiency of the immune system acquired during an individual’s lifetime, not a congenital disease
  • The first reports of AIDS were in 1981, and it has since spread globally, killing over 25 million people
  • AIDS is caused by the Human Immuno deficiency Virus (HIV), a retrovirus group with an envelope enclosing the RNA genome
  • HIV is transmitted through sexual contact with an infected person, transfusion of contaminated blood, sharing infected needles, and from infected mother to child through the placenta
  • People at high risk include those with multiple sexual partners, intravenous drug users, those needing repeated blood transfusions, and children born to an HIV-infected mother
  • HIV is not spread through mere touch or physical contact, only through body fluids
  • The period between HIV infection and appearance of AIDS symptoms varies from a few months to many years (usually 5-10 years)
  • After entering the body, the virus replicates in macrophages and helper T-lymphocytes, leading to a decrease in the number of helper T-lymphocytes and an increase in infections
  • A widely used diagnostic test for AIDS is the enzyme linked immuno-sorbent assay (ELISA)
  • Treatment with anti-retroviral drugs can only prolong life, not prevent death
  • Prevention is the best option for AIDS as it has no cure, and is often spread through conscious behavior
  • The National AIDS Control Organization (NACO) and NGOs in India, as well as WHO, are working to educate people and prevent the spread of HIV
  • Infection with HIV or having AIDS should not be hidden, as it can spread to others
  • HIV/AIDS-infected people need help and sympathy, not isolation, to tackle the disease
  • The disease can only be prevented by society and the medical community working together.


  1. Cancer is a leading cause of death globally and a large number of people suffer from it, especially in India.
  2. The development and growth of cancer is caused by the breakdown of regulatory mechanisms of cell growth and differentiation.
  3. Cancer cells have lost the property of contact inhibition and continue to divide, leading to tumor formation.
  4. Tumors can be benign or malignant.
  5. Malignant tumors are a mass of rapidly proliferating cells that invade and damage surrounding normal tissues, causing harm by competition for nutrients and metastasis.

Causes of cancer

  1. Transformation of normal cells into cancerous neoplastic cells may be induced by physical, chemical, or biological agents (carcinogens).
  2. Ionizing radiations (X-rays, gamma rays) and non-ionizing radiations (UV) cause DNA damage leading to neoplastic transformation.
  3. Chemical carcinogens present in tobacco smoke are a major cause of lung cancer.
  4. Cancer-causing viruses (oncogenic viruses) have genes called viral oncogenes.
  5. Normal cells contain genes called cellular oncogenes (c-onc) or proto oncogenes, which when activated, can lead to oncogenic transformation.

Cancer detection and diagnosis

  1. Early detection of cancers is essential for successful treatment in many cases.
  2. Cancer detection is based on biopsy and histopathological studies of tissue and increased cell counts in the case of leukemias.
  3. Techniques like radiography (X-rays), CT (computed tomography), and MRI (magnetic resonance imaging) are used to detect internal organ cancers.
  4. Antibodies against cancer-specific antigens and molecular biology techniques can also be used for cancer detection.
  5. Identification of genes that predispose an individual to certain cancers can aid in prevention and avoiding exposure to carcinogens.

Treatment of Cancer

Common approaches for treatment of cancer are 1. Surgery, 2. Radiation therapy, and 3. Immunotherapy. But, Most cancers are treated with a combination of surgery, radiotherapy, and chemotherapy.

  1. In Radiotherapy, tumor cells are lethally irradiated while taking care of surrounding normal tissues.
  2. Chemotherapeutic drugs are used to kill cancerous cells, some of which are specific for particular tumors with side effects like hair loss and anemia.
  3. Tumor cells avoid detection and destruction by the immune system, so patients are given biological response modifiers (e.g. α-interferon) to activate their immune system and help destroy the tumor.

Drugs and Alcohol Abuse

  • Use of drugs and alcohol, particularly among youth, has been increasing.
  • Commonly abused drugs include opioids, cannabinoids, and coca alkaloids.
  • Opioids bind to specific receptors in the central nervous system and Gastrointestinal tract. [Ques. Where in the Human Body Are Its Specific Receptors Located ? How Do Opioids Affect the Human Body?]
  • Heroin, chemically diacetylmorphine, is a depressant obtained from the latex of the poppy plant (Papaver somniferum).
  • Cannabinoids interact with receptors in the brain and are obtained from the inflorescences of the cannabis plant.
  • Coca alkaloid or cocaine is obtained from the coca plant and interferes with the transport of the neurotransmitter dopamine.
  • Barbiturates, amphetamines, benzodiazepines, and other similar drugs are often abused.
  • Plants, fruits, and seeds with hallucinogenic properties have been used for centuries in folk medicine, religious ceremonies, and rituals.
  • Smoking is a common form of substance abuse and is associated with increased risk of various cancers, bronchitis, emphysema, heart disease, gastric ulcer, and more.
  • Tobacco contains nicotine and other chemical substances that increase blood pressure and heart rate.
  • Smoking increases CO content in the blood and reduces oxygen concentration, leading to oxygen deficiency in the body.
  • Counseling and medical help are necessary for those addicted to drugs or tobacco.

Adolescence and Drug/Alcohol Abuse

  1. Adolescence is a period of transition from childhood to adulthood, lasting approximately from 12-18 years of age.
  2. Adolescence is marked by both biological and behavioral changes.
  3. It is a vulnerable phase of mental and psychological development.
  4. Curiosity, need for adventure and excitement, and experimentation are common reasons for adolescent drug and alcohol use.
  5. Stress and peer pressure can also play a role in adolescent substance abuse.
  6. The perception that drug and alcohol use is “cool” or progressive is promoted by media, and can influence adolescent behavior.
  7. Unstable or unsupportive family structures and peer pressure are other factors associated with adolescent substance abuse.

Addiction and Dependence

  1. Drugs and alcohol are frequently used due to perceived benefits such as euphoria and a temporary feeling of well-being.
  2. Addiction is a psychological attachment to the effects of drugs and alcohol.
  3. With repeated use of drugs, the tolerance level of receptors increases, leading to greater intake and addiction.
  4. Addiction can occur even from the first use of drugs or alcohol.
  5. Dependence is the body’s tendency to manifest unpleasant withdrawal symptoms if regular drug/alcohol use is abruptly discontinued.
  6. Withdrawal symptoms can be severe and may require medical supervision.
  7. Dependence leads to ignoring social norms to obtain funds, causing social adjustment problems.

Effects of Drug/Alcohol Abuse

  • Immediate adverse effects of drugs and alcohol abuse include reckless behavior, vandalism, violence, and potentially coma or death.
  • Overdosing can occur from taking drugs or combining them with alcohol.
  • Warning signs of drug and alcohol abuse among youth include: drop in academic performance, lack of interest in personal hygiene, withdrawal, isolation, depression, aggressive behavior, deterioration in relationships, changes in sleeping and eating habits, and fluctuations in weight and appetite.
  • Drug/alcohol abuse can lead to stealing and mental and financial distress for the user and their family and friends.
  • IV drug use increases the risk of acquiring serious infections such as AIDS and Hepatitis B.
  • Alcohol abuse during adolescence can lead to heavy drinking in adulthood and damage the nervous system and liver.
  • Drugs and alcohol abuse during pregnancy can harm the fetus.
  • Some athletes misuse drugs to enhance performance, with side effects including masculinization, acne, aggressiveness, mood swings, depression, stunted growth, and permanent changes to sexual characteristics and organ function.

Prevention and Control

  • The adage “prevention is better than cure” applies to substance abuse.
  • Adolescents are more likely to start using drugs, alcohol, or tobacco.
  • Parents and teachers have a responsibility to prevent substance abuse in adolescents.
  • Parenting with high levels of nurturance and discipline can lower the risk of substance abuse.

Ways to prevent substance abuse in adolescents include: Avoiding peer pressure, education and counseling, seeking help from parents and peers, looking for danger signs, and seeking professional and medical help.

  1. Avoid undue peer pressure
  • Every child has their own choice and personality that should be respected and nurtured
  • Avoid pushing the child beyond their limits in studies, sports, or other activities
  1. Education and Counselling
  • Teach the child to face problems and stress, and to accept disappointments and failures as a part of life
  • Channel their energy into healthy pursuits such as sports, reading, music, yoga, and other extracurricular activities
  1. Seeking Help from Parents and Peers
  • Seek immediate help from parents, peers, or trusted friends
  • Get proper advice to solve problems and vent feelings of anxiety and guilt
  1. Looking for Danger Signs
  • Alert parents, teachers, and friends need to look for and identify danger signs of drug/alcohol abuse
  • Bring it to the attention of parents, teachers, or medical professionals for proper diagnosis and treatment
  1. Seeking Professional and Medical Help
  • A lot of help is available in the form of psychologists, psychiatrists, and rehabilitation programs
  • With sufficient effort and willpower, affected individuals can overcome the problem and lead a normal, healthy life.