Class 12 Biology Ch 8 Human Health And Disease Notes PDF

Health is more than just the absence of illness. According to the World Health Organization (WHO), it is a state of complete physical, mental, and social well-being.

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Good health depends on several factors:

• Genetic factors
• Environmental factors
• Lifestyle habits
• Socio-economic conditions

When the body is disturbed by structural or functional problems, we say that the person has a disease. Diseases can be classified broadly into infectious and non-infectious. Get quick summary for CBSE Class 12 Biology Syllabus.

Health is defined by the World Health Organization (WHO) as a state of complete physical, mental and social well-being, and not merely the absence of disease.

Common Diseases in Humans

These are some common diseases in humans.

Infectious Diseases

Infectious diseases are caused by pathogens, microorganisms like bacteria, viruses, fungi, protozoa, and helminths. They spread through different means such as air, water, vectors, or direct contact.

A. Bacterial Diseases

• Typhoid
  
  • Causative agent: Salmonella typhi
  • Transmission: Through contaminated food and water (faeco-oral route)
  • Symptoms: Sustained high fever (up to 104 °F), weakness, abdominal pain, headache, and loss of appetite. In severe cases, intestinal perforation and death may occur
  • Diagnosis: Widal test detects antibodies against the bacteria
  • Prevention: Sanitation, clean drinking water, and vaccination
• Pneumonia
  
  • Causative agent: Streptococcus pneumoniae, Haemophilus influenzae
  • Transmission: Droplet infection from cough or sneeze of an infected person
  • Symptoms: Fever, chills, cough, difficulty breathing, and chest pain
  • Prevention: Vaccination, avoiding overcrowded places, and maintaining hygiene

B. Viral Diseases

• Common Cold
  
  • Causative agent: Rhinoviruses
  • Transmission: Airborne droplets and contaminated objects
  • Symptoms: Nasal congestion, sore throat, cough, tiredness, and headache
  • Prevention: Good hygiene and avoiding close contact with infected individuals
• Dengue & Chikungunya
  
  • Causative agents: Dengue virus and Chikungunya virus
  • Transmission: Bite of Aedes aegypti mosquito
  • Symptoms (Dengue): High fever, severe headache, muscle/ joint pain, skin rash
  • Symptoms (Chikungunya): High fever, severe joint pain lasting weeks
  • Prevention: Avoid mosquito breeding (stagnant water), use mosquito nets and repellents

C. Protozoan Diseases

• Malaria
  
  • Causative agent: Plasmodium vivax, P. falciparum, P. malariae, P. ovale
  • Transmission: Bite of infected female Anopheles mosquito
  • Life cycle: The parasite multiplies in human liver and red blood cells, and in mosquito gut
  • Symptoms: Periodic fever with chills and sweating. P. falciparum causes cerebral malaria,  the most severe form
  • Diagnosis: Blood smear examination
  • Prevention: Mosquito control, insecticide-treated nets, prophylactic drugs
• Amoebiasis (Amoebic Dysentery)
  
  • Causative agent: Entamoeba histolytica
  • Transmission: Through contaminated food and water
  • Symptoms: Abdominal pain, cramps, stools with mucus and blood
  • Prevention: Proper sanitation and clean water

D. Helminthic Diseases

• Ascariasis
  
  • Causative agent: Ascaris lumbricoides (roundworm)
  • Transmission: Ingestion of eggs in contaminated food/ water
  • Symptoms: Abdominal pain, indigestion, intestinal blockage in severe cases
  • Prevention: Sanitation and washing raw vegetables/ fruits
• Filariasis (Elephantiasis)
  
  • Causative agent: Wuchereria bancrofti, Wuchereria malayi
  • Transmission: Bite of infected Culex mosquitoes
  • Symptoms: Chronic swelling of legs, scrotum, or breasts due to lymphatic blockage
  • Prevention: Mosquito control and mass drug administration

E. Fungal Diseases

• Ringworm
  
  • Causative agents: Microsporum, Trichophyton, Epidermophyton
  • Transmission: Direct contact or contaminated articles (towels, combs)
  • Symptoms: Circular red itchy patches on skin, scalp, or nails
  • Prevention: Personal hygiene and avoiding sharing clothes/ towels

Basic Concepts of Immunology

Immunity is the body’s ability to resist harmful microorganisms. They can be classified into the following:

• Active immunity: Developed after infection or vaccination; long-lasting.
• Passive immunity: Transfer of ready-made antibodies (e.g., mother to infant via breast milk); short-term.

Innate Immunity

Present from birth; non-specific defence. Includes:

• Physical barriers: Skin, mucous membranes.
• Physiological barriers: Acid in stomach, tears, saliva.
• Cellular barriers: Neutrophils, monocytes, macrophages, natural killer cells.
• Cytokine barriers: Interferons protect against viral infections.

Acquired Immunity

Develops after birth; specific to pathogens. It involves B-lymphocytes (antibody production) and T-lymphocytes (cell-mediated immunity). These are the types of acquired immunity:

• Humoral Immunity - mediated by B-lymphocytes, produces antibodies
  
• Cell-mediated Immunity - mediated by T-lymphocytes

Vaccination and Immunisation

Vaccination uses inactivated/ weakened pathogens or their products to stimulate immunity without causing disease. Immunisation refers to the process of developing immunity.

Examples:

• BCG → Tuberculosis
• OPV → Polio
• MMR → Measles, Mumps, Rubella
• Hepatitis B vaccine

Allergies

An overreaction of the immune system to harmless substances (allergens). It is Caused by release of histamine from mast cells (mediated by IgE antibodies).

• Symptoms: Sneezing, watery eyes, rash, breathing difficulty.
• Treatment: Antihistamines and steroids.

Autoimmunity

Autoimmunity is a condition where the immune system attacks its own tissues, causing chronic inflammation and damage. For example, Rheumatoid arthritis, Multiple sclerosis, etc.

Immune System Disorders

Immune disorders, which involve a dysfunctional immune system, are generally classified as overactive (autoimmune) or underactive (immunodeficiency). Common types include:

AIDS (Acquired ImmunoDeficiency Syndrome)

• Causative agent: Human Immunodeficiency Virus (HIV)
• Transmission: Unprotected sex, contaminated needles, infected blood transfusion, mother-to-child
• Effect: Destroys helper T-lymphocytes, leading to immune suppression
• Prevention: Safe sex, screened blood, disposable needles
• Treatment: No cure; managed with antiretroviral therapy (ART)

Cancer

• Nature: Uncontrolled cell division forming malignant tumours
• Spread: Metastasis (secondary tumour formation)
• Causes: Gene mutations, carcinogens, viruses (oncoviruses), radiation
• Treatment: Surgery, chemotherapy, radiotherapy, immunotherapy

Drugs and Alcohol Abuse

Affects health, behaviour, and society. Common abused substances:

• Opioids: Heroin – depresses brain activity
• Cannabinoids: Marijuana – affects thinking and memory
• Coca alkaloids: Cocaine – stimulates CNS

Alcohol abuse damages the liver, nervous system, and impairs judgment.

Prevention: Education, counselling, community programs.

Conclusion

Human Health and Disease explains the causes, transmission and prevention of major diseases along with the working of the immune system.

Understanding immunity, vaccination, AIDS, cancer and substance abuse is essential for both board examinations and competitive exams like NEET. The chapter emphasises that prevention through awareness and hygiene is more effective than cure.

FAQs

Q1. What is innate immunity?

Ans. Non-specific defence present from birth.

Q2. What is the difference between humoral and cell-mediated immunity?

Ans. Humoral → antibodies (B cells) | Cell-mediated → T cells

Q3. What is metastasis?

Ans. Spread of cancer cells to secondary sites.

Q4. How does HIV affect the immune system?

Ans. It destroys helper T-cells (CD4), weakening immunity.

Q5. What is vaccination?

Ans. Introduction of antigen to stimulate immune memory.