Human Health and Diseases Revision Notes

Class 12 Notes

Please refer to the Human Health and Diseases Revision Notes given below. These revision notes have been designed as per the latest NCERT, CBSE and KVS books issued for the current academic year. Students will be able to understand the entire chapter in your class 12th Biology book. We have provided chapter wise Notes for Class 12 Biology as per the latest examination pattern.

Revision Notes Chapter 8 Human Health and Diseases

Students of Class 12 Biology will be able to revise the entire chapter and also learn all important concepts based on the topic wise notes given below. Our best teachers for Grade 12 have prepared these to help you get better marks in upcoming examinations. These revision notes cover all important topics given in this chapter.

  • Health is defined as a state of complete physical, mental and social well being and not merely the absence of disease or infirmity. Good health is a state of optimum physical fitness, mental maturity and alertness with freedom from anxiety and social well being with freedom from social tensions.
  • When the functioning of one or more organs or systems of the body is adversely affected, characterised by various signs and symptoms, we say that we are not healthy, i.e., we have a disease.
Human Health and Diseases Revision Notes

Communicable Human Diseases

Human Health and Diseases Revision Notes
Human Health and Diseases Revision Notes
Human Health and Diseases Revision Notes
Human Health and Diseases Revision Notes


  • Malaria is caused by a digenetic (having two hosts to complete its life cycle) and triphasic (having three phases of life cycle) protozoan parasite known as Plasmodium. Sir Ronald Ross (1897), a doctor in Indian Army, established that malarial parasite is transmitted by the bite of a female Anopheles mosquito for which he got Nobel Prize in 1902.
  • Life cycle of Plasmodium requires two hosts for completion human and mosquito. Plasmodium enters the human body as sporozoites (infectious form) through the bite of infected female Anopheles (vector). The sporozoites reach the liver cells via blood where they initially multiply.
  • These then attack the RBCs resulting in their rupturing. The rupture of RBC is associated with the release of haemozoin, a toxin which causes the chill and high recurring fever every three to four days.
  • The female Anopheles mosquito when bites an infected human being, the malarial parasites enter into the mosquito’s body and undergo further development to form sporozoites that finally move to the salivary glands of the insect. 
  • The bite of these mosquitoes introduces the sporozoites inside the human body, thus initiating the above mentioned cyclic process again.
  • The attack of malaria is preceded by tiredness, headache and muscular pain. Malaria is characterised by recurring rigors lasting 6-10 hours. There are three stages:
    (i) cold stage (chill and shivering).
    (ii) hot stage (high fever).
    (iii) sweating stage (perspiration and gradual fall in temperature).
  • There are four species of Plasmodium which cause four main types of malaria in humans. They are :
    Plasmodium vivax : Causes benign tertian malaria in which fever recurs after every 48 hours.
    P. malariae : Causes quartan malaria in which fever appears after every 72 hours, and often produces persistent subclinical malaria.
    P. falciparum : Causes cerebral malaria or malignant tertian malaria where fever recurs after every 48 hours.
    P. ovale : Causes mild tertian malaria.


  • Diabetes mellitus or hyperglycemia is the most common endocrine disorder of the pancreas caused when either insulin is produced insuciently (non-insulin dependent diabetes mellitus) or become non functional (insulin dependent diabetes mellitus). 
  • In both disorders, the blood glucose concentration is elevated above the normal range. Glucose is excreted in urine, excessive urination and dehydration of body tissues occurs.
  • Cardiovascular diseases are those diseases that affect the blood vessels and heart. E.g., arteriosclerosis (hardening and loss of elasticity of arteries), atherosclerosis (development of lumpy thickness on inner walls of arteries that prevent their dilation), hypertension (high blood pressure), angina pectoris (chest pain due to sclerosis of coronary arteries) etc.
  • Cerebrovascular accident (CVA) or stroke is the sudden interruption of blood flow to a portion of the brain because of block or rupture of a cerebral blood vessel. Thus the brain cells do not get oxygen and glucose. This can cause paralysis, loss of speech etc.
  • Arthritis is a general term that is applied to different malfunctions of the joints. These could be osteoarthritis (progressive erosion of articular cartilage at synovial joint), rheumatoid arthritis (inflammation of synovial membrane in synovial joints resulting in formation of hard tissues over cartilage articulations) infectious arthritis (due to bacterial and viral infection)and gouty arthritis (accumulation of uric acid crystals in joints)


  • The overall ability of an organism to fight with the disease-causing organisms, conferred by the immune system is called immunity.
  • The human immune system comprises lymphoid organs, tissues and soluble molecules such as antibodies.
  • Lymphoid organs are those organs where the maturation and proliferation of lymphocytes takes place. The structurally and functionally diverse lymphoid organs and tissues are interconnected by the blood vessels and lymphatic vessels through which lymphocytes circulate.
Human Health and Diseases Revision Notes
  • Antibodies are immunoglobulins (Igs) which are produced in response to antigenic stimulation.
  • Immunoglobulins are glycoproteins made up of four polypeptide chains (linked by disulphide bonds)—two heavy (H) and two light (L). Light and heavy chains are subdivided into variable and constant regions.
Human Health and Diseases Revision Notes

Immune Response

  • The specific reactivity induced in a host by an antigenic stimulus is known as the immune response. It is of two types – primary and secondary immume response.
  • The reaction of the body’s immune system to the first attack of microbes (antigens) is called primary immune response. It takes much longer time to develop because of the requirement of suitable receptor development.
  • The reaction of the body’s immune system to any subsequent infection of the same microbe is termed secondary immune response. This response is quicker and more intense than the primary immune response.

Types of immunity or defence mechanism  

Human Health and Diseases Revision Notes
  • T-lymphocytes or T-cells when triggered by antigens, produce a clone of T-cells such as :
    1. Cytotoxic or Killer T-cells directly attack and destroy antigens by secreting a protein perforin.
    2. Helper T-cells stimulate the B-cells to produce antibodies. They form protein mediators called lymphokines.
    3. Suppressor T-cells keep a check on entire immune system from attacking the body’s own cells.
    4. Memory T-cells are sensitised T-cells which retain memory of antigen specificity for future, sometimes life long.
  • B-lymphocytes or B-cells produce antibodies and therefore, generate antibody mediated or humoral immunity. They give rise to the following cells :
    Plasma cells (Effector B-cells) : The antigen specific T-cells stimulate specific B-lymphocytes to multiply rapidly forming clone of plasma cells.
    Memory B-cells : Some of the activated B cells do not differentiate into plasma cells but rather remain dormant as memory B cells until  activated once again by a renewed attack of the same antigen.
  • Antigen presenting cells (APCs) are specialised cells which include macrophages, B-lymphocytes and dendritic cells (e.g., Langerhans cells of epidermis of skin). They are distinguished by two properties :
    (i) they express class II MHC molecules on the membrane, and
    (ii) they are able to deliver a costimulatory signal that is necessary for helper T cell activation.

Vaccination and Immunisation

  • Vaccination (L. vacca-cow) is the process of development of immunity against a particular disease by inoculation of harmless antigenic material like attenuated pathogen or its toxoid into a healthy person.
  • Vaccine is a suspension of killed or attenuated pathogenic microorganisms or antigenic preparations made out of it, which on administration provides immunity towards the pathogen.
  • Immunisation is the process by which the body produces antibodies against the antigens of various diseases through administration of specific vaccines.   
Human Health and Diseases Revision Notes

Disorders of Immune System
1. Hypersensitivity (Allergy)

  • Hypersensitivity or allergy is the excessive immune response to common antigens, called allergens.
  • Common examples of allergens are mites in dust, pollens, etc. Symptoms of allergic reactions include sneezing, watery eyes, running nose and diculty in breathing.
  • Allergy is due to the release of chemicals like histamine and serotonin from the mast cells.
Human Health and Diseases Revision Notes
  • Hay fever, asthma, anaphylactic shock are some common examples of allergy.

2. Autoimmunity:
Autoimmunity refers to abnormal immune responses in which the immune system fails to properly distinguish between self and non-self and attacks self body part. Examples of autoimmune disorders are rheumatoid arthritis, insulin dependent diabetes, chronic anaemia, chronic hepatitis, Grave’s disease etc.


  • AIDS or acquired immune deficiency syndrome is a deficiency of immune system. It is accompanied by a reduction in the number of helper T cells which stimulate antibody production by B-cells resulting in loss of natural defence against viral infection.
  • HIV is a retrovirus that attacks helper T cells.
  • Virus of AIDS is transmitted by transfusion of infected blood or blood products, use of contaminated needles and syringes to inject drugs or vaccines, sexual intercourse with an infected partner without a condom and from infected mother to child through placenta.
  • The incubation period of AIDS ranges between 6 months to 10 years. AIDS can be diagnosed by ELISA test and Western blotting test. Western blotting test is employed for confirmation of ELISA positive cases.
  • No vaccine has been prepared so far against AIDS virus.

Organ transplantation
It is the implantation of an organ or tissue from one part of the body to another or from one person (donor) to another (recipient). Tissue matching, blood group matching are essential before undertaking any graft /transplant and even after this the patient has to take immunosuppresants all his/her life. This is because the body is able to differentiate ‘self ’ and ‘nonself ’ and the cell-mediated immune response is responsible for the graft rejection.


  • Cancer is a disease of uncontrolled proliferation of cells without any differentiation. It commonly originates in the tissues in which the cells are regularly replaced by mitosis. 
  • In our body, cell growth and differentiation is highly controlled and regulated. In cancer cells, there is breakdown of these regulatory mechanisms. Normal cells show a property called contact inhibition, by virtue of which contact with other cells inhibits their uncontrolled growth. Cancer cells appear to have lost this property. As a result of this, cancerous cells just continue to divide giving rise to masses of cells called tumours. Tumour is of two types :
    Benign tumour : This is a solid tumour that stops growing by itself and does not invade other tissues and remains confined to a particular site.
    Malignant or tumour : In malignant tumours, tumour cells grow very rapidly, invading and damaging the surrounding normal tissues. Cells from such tumours reach distant sites through blood, and whenever they get lodged in the body, they start a new tumour there. This property called metastasis is the most feared property of malignant tumours. 
  • Chemical and physical agents that can cause cancer are called carcinogens. These include 
    (i) over exposure to ionising radiations like X-rays, UV rays, gamma rays, etc
    (ii) Chemicalslike steroids, and arsenic air pollutants
    (iii) Viruses, viz. human papilloma virus, the human T cells lymphocytic virus, and hepatitis B virus
    (iv) oncogenes and tumour suppressor genes.
  • Techniques such as radiography (use of X-rays), CT scan (computed tomography), MRI scan (magnetic resonance imaging) are very useful to detect cancers of the internal organs. Mammography is radiographic examination of breasts for possible cancer.


  • Drug addiction or drug abuse is taking drugs for purpose other than clinical use, in amount, concentration or frequency that impairs physical, physiological and psychological functions of the body. The drugs, which arecommonly abused are opioids, coca alkaloids and cannabinoids. Majority of these are obtained from flowering plants. Some are obtained from fungi.
  • Habituating drugs are of two types : psychotropic and psychodelic. Psychotropic drugs include tranquillizers, sedatives, hypnotics, opioids and stimulants. 
    1. Tranquillizers decrease tension and anxiety and produce feeling of calmness
    2. Sedatives reduce excitement, assuage pain and induce drowsiness.
    3.Hypnotics also induce sleep.
  • Opiate narcotics/Opioids are the drugs derived from dried latex of unripe capsular fruits of poppy plant, Papaver somniferum. Opioids bind to specific opioid receptors present in our central nervous system and gastrointestinal tract. They are also called pain killers. Opiates have narcotic, analgesic, and sedative effect. 
Human Health and Diseases Revision Notes
  • Stimulants stimulate nervous system, and cause excitement. These are caffeine, cocaine, crack, amphetamines etc. 
  • Psychedelic drugs change one’s behaviour, thoughts and perception etc. These are LSD, mescaline, prilocybin, cannabinoids, etc.
  • An adolescent can fall easy prey to drugs, tobacco and alcohol. The various reasons are  curiosity, experimentation, adventure and excitement, group or peer pressure, feeling of independence, overcoming frustration and depression and unsupportive family structure.

Effects/Symptoms of Drug Abuse 

  • Reckless behaviour, and violence. 
  • Coordination of body parts, working of nervous and muscular systems are inuenced.
  • Disturbed peristalsis and secretion of digestive enzymes.
  • Effect on liver and kidneys as they become involved in metabolism and elimination of their products. The damaged liver undergoes cirrhosis.
  • Arrhythmia, bradycardia and tachycardia develop impotency.
  • Taking of drugs intravenously may spread serious infections like AIDS and hepatitis B due to sharing of infected needles and syringes.
  • Lack of concentration.
  • Mental and financial distress to the entire family.

Prevention and Control

  • Avoid undue pressure on child.
  • Teachers and parents should always be careful to look for and identify danger signs that can indicate tendency to go in for addiction.
  • Psychologists, psychiatrists, deaddiction and rehabilitation specialists are available to help inidviduals who fall in the trap of drug, tobacco or alcohol abuse.
Human Health and Diseases Revision Notes