Human Health and Disease Class 12 Notes Biology Chapter 8


  • Health is a state of complete physical, social, and mental well-being and not merely the absence of disease or infirmity.
  • Disease is any condition that interferes with the normal structure & function of the body that is manifested by a characteristic set of symptoms & sign . Infection is the process by which harmful microorganism enters the body and makes them ill or diseases.
  • Contamination refers to the presence of disease-causing agents in nonliving things like water, food, etc.
  • Infestation is the presence of an unusually large number of insects or animals in a place, typically so as to cause damage or disease.
  • Diseases are of two types on the basis of their occurrence: congenital and acquired. Congenital diseases are inborn diseases whereas acquired disease occurs only after birth. They are further of two types: communicable and non-communicable.
  • Communicable (infectious) diseases are caused by viruses, rickettsia, bacteria, fungi, protozoans and worms and non-communicable (non-infectious) diseases remain confined to the person who develops them and does not spread to others.
  • Diseases caused by bacteria are typhoid, pneumonia, dysentery, plague, diphtheria, etc.
  • Typhoid is caused by the bacterium Salmonella typhi. Symptoms are high fever, headache, weakness, loss of appetite, skin rash, constipation, stomach ache etc. Typhoid is diagnosed with a widal test.
  • Pneumonia is an infection of lung which is caused by bacteria, virus, fungi or parasites. It is a type of chest infection which affects the tiny air sacs (called alveoli) in your lungs. In this condition these air sacs get inflamed and fill with fluid which makes it harder to breathe. It is characterised by difficulty in respiration, fever, chills, cough, and headache. It is caused by bacteria like Streptococcus pneumoniae and Haemophilus influenza.
  • Diseases caused by virus are common cold, small pox, measles, mumps, poliomyelitis, dengue, yellow fever, rabies etc.
  • Rhinoviruses causes one of the most infectious disease called common cold. Droplets resulting from cough or sneezes of an infected person are either inhaled directly or transmitted through contaminated objects.
  • Diseases caused by protozoans are malaria, amoebiasis, giardiasis, Kala azar, trypanosomiasis etc.
  • Malaria
    • Malaria is caused by Plasmodium species & spread through female Anopheles mosquito. Primary host are female mosquito of genus Anopheles & humans acts as intermediate host.
    • Plasmodium enters the human body as sporozoites (infectious form) through the bite of infected female Anopheles mosquito.
  • Amoebic dysentery or amoebiasis
    • It is caused by Entamoeba histolytica.
    • It is transmitted by poor sanitation and contamination of drinking water or food by faeces of infected persons. Houseflies acts as mechanical carriers & serve to transmit the parasite from faeces of infected person to food & its products thereby contaminating them.
  • Filariasis is caused by Wuchereria bancrofii. It is also known as elephantiasis due to excessive enlargement of body parts like legs.
  • Ascariasis
    • It is caused by Ascaris lumbricoides and it is more common in children.
    • Vectors for ascariasis are flies or cockroaches containing embryonated eggs of Ascaris contaminated with food & water.
    • Ascariasis is characterised by colic pain, fever, indigestion, diarrhoea, vomiting, weakness, retarded physical & mental efficiency, internal bleeding, muscular pain, blockage of intestinal passage.
  • Fungi causes diseases and these are known as mycosis. Microsporum, Trichophyton and Epidermophyton are responsible for ringworms, which is characterised by appearance of dry scaly lesions of the skin, nails and scalps.
  • Cancer is a disease caused by an uncontrolled division of abnormal cells in a part of the body.
  • Cancer can sometimes spread to other parts of the body – this is called a secondary tumour or a metastasis.
  • Cancer and its treatments can affect body systems, such as the blood circulation, lymphatic and immune systems, and the hormone system.
  • Tumours are usually divided into benign and malignant. A benign tumour is localised, develops slowly and does not usually result in the patient’s death. Malignant or cancerous tumours develop more rapidly. They are not localised and are often fatal for the patient.
  • Carcinogens are factors or physical or chemical agents which induces the cancer growth. E.g. coal tar, asbestos, cadmium oxides, etc. Cancer causing viruses are called oncogenic virus. They have genes called viral oncogenes. In contrast, several genes called proto-onco genes or cellular oncogenes (c-onc) have been identified in normal cells which, when activated under certain conditions, could lead to oncogenic transformations of the cells
  • AIDS (Acquired Immuno Deficiency Syndrome) is a chronic life threatening disorder which damages the human body’s immune system.
  • It is caused by Human Immuno Deficiency Virus (HIV) which is retrovirus having envelope enclosing the RNA genome.
  • The major cell affected by HIV is the helper T-lymphocytes.
  • The HIV multiplies & slowly begins todestroy the CD4 lymphocytes (T cells or helper cells) a type of white blood cells (WBC) which is responsible for the
  • coordination of the entire immune system.
  • AIDS occurs when CD4 cell count falls below 200 cells/mm3 and vulnerable to opportunistic infections.
  • AIDS is diagnosed by ELISA (Enzyme Linked Immuno Sorbent Assay). Commonly used enzyme is ELISA techniques are peroxidases and alkaline phosphate.
  • The medicine used to treat HIV is called antiviral therapy or ART.
  • The system of human body which protects it from various infectious agents & cancer is known as immune system.
  • Immunity against infectious disease is of two types : innate immunity and acquired immunity.
  • Innate immunity is developed in an individual without having the disease or immunisation, e.g., recreation of sweat glands contain certain chemical substances which prevent the entry of micro-organisms. It is also called non­ specific defence mechanism. It is present from birth.
  • The resistance against infectious disease that an individual acquires during life is known as acquired immunity.
  • Acquired immunity occurs after exposure to an agent, is specific and is mediated by antibody and T lymphocyte (T cells, cytotoxic cells).
  • Active acquired immunity may be natural or artificial.
  • In natural active acquired immunity, antibody production is stimulated by living organisms or their products which cause the disease.
  • The resistance that is transmitted to recipient in a ready-made form is known as passive acquired immunity. There is no antigenic stimulus. There is no active role of immune system of the recipient. The immunity is transient, lasting for a few days or weeks.
  • Immunoglobulin is blood proteins, produced by plasma cells as an immune response against foreign agents (antigens).
  • Inoculation of vaccines to prevent diseases is called immunisation. Vaccines are preparations of living or killed micro-organisms or their products. Vaccines are of two types – live vaccines and killed vaccines.
  • Genetic vaccines involve one or more genes from diseases carrying agent (pathogen) and splicing these gene into plasmids (closed rings of DNA).
  • Allergy is a hypersensitivity reaction acquired through exposure to a particular allergens that can involves various deleterious effects such as Hay fever, asthma, serum sickness, systemic anaphylaxis.
  • Drugs are classified on the basis of mode of action on brain into 2 categories:
    • Psychotropic or psychoactive drugs (mood altering or neurological drugs).
    • Psychedelic or hallucinogenic drugs.
  • Commonly abused drugs are opioids, cannabinoids and coca, alkaloids. Nicotine acts as tranquilisers but also stimulates the release of adrenaline leading to high blood pressure.
  • Adolescence is the period of rapid growth, the physical & mental development poised between childhood & adulthood (between 8-18 years for girls and 7- 19 years of boys).
  • Addiction refers to the dependence on and craving for tobacco, alcohol or a particular drug.
  • Alcoholism is the consumption of or pre-ccupation with alcoholic beverages to the extent that this behavour interferes with the alcoholic’s normal personal, family life.
  • Alcohol decreases the activity of CNS thereby reducing anxiety, tensions & inhibitions. It also decreases the ADH secretion causing increased urine output.
  • Excessive doses of drugs may lead to coma & death due to respiratory, heart failure & cerebral haemorrhage.

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