Human Health and Disease Class 12 Notes

Common Diseases and Their Pathogens

Communicable diseases are caused by pathogens — bacteria, viruses, protozoans, helminths and fungi — transmitted through contaminated food, water, air or vectors.

• Typhoid: caused by Salmonella typhi, transmitted through contaminated food and water. Diagnosis: Widal test (detects antibodies against H and O antigens). Symptoms include sustained high fever and abdominal pain.
• Pneumonia: infection of the lungs caused by Streptococcus pneumoniae or Haemophilus influenzae. Spread by droplet infection. Symptoms: cough, fever, difficulty breathing. Prevented by pneumococcal vaccine.
• Common cold: caused by rhinoviruses (over 100 serotypes). Airborne droplet transmission. Symptoms: sneezing, nasal congestion, sore throat. No vaccine available.
• Amoebiasis (amoebic dysentery): caused by Entamoeba histolytica (protozoan), spread through contaminated food and water. Affects the large intestine.
• Ascariasis: caused by Ascaris lumbricoides (roundworm). Transmitted through ingestion of eggs in contaminated food, water or soil.
• Filariasis (elephantiasis): caused by Wuchereria bancrofti and Wuchereria malayi (filarial worms), transmitted by Culex mosquito. Affects lymphatic vessels.
• Ringworm: caused by dermatophyte fungi (Trichophyton, Microsporum, Epidermophyton). Spread by direct contact and contaminated objects.

Plasmodium vivax life cycle: Sporozoites from mosquito bite → liver cells (exo-erythrocytic schizogony) → merozoites released into blood → infect RBCs (erythrocytic schizogony) → RBCs rupture releasing merozoites and toxins → periodic fever every 48 hours. Some merozoites develop into gametocytes, which are picked up by a female Anopheles mosquito to continue the cycle.

Immunity — Types and Mechanisms

• Innate immunity: physical barriers (skin, mucous membranes), chemical barriers (lysozyme, stomach acid), cellular defences (neutrophils, macrophages, natural killer cells) and inflammatory response.
• Acquired immunity: develops after exposure to antigens. Characterised by specificity and memory. Two types — humoral (antibody-mediated) and cell-mediated.
• Active immunity: the body produces its own antibodies after exposure to an antigen (natural infection or vaccination). Provides long-lasting memory.
• Passive immunity: ready-made antibodies are introduced from outside (mother's milk IgA, antiserum/antitoxin). Provides immediate but temporary protection.

Vaccination, Allergies and Autoimmunity

• DPT vaccine: Diphtheria, Pertussis (whooping cough), Tetanus — given as a cocktail.
• Polio vaccine: Salk (IPV, injectable) and Sabin (OPV, oral — live attenuated).
• BCG vaccine: for tuberculosis — live attenuated Mycobacterium bovis.
• Hepatitis B vaccine: recombinant vaccine using HBsAg (surface antigen) produced in yeast.

Cancer — Causes and Detection

• Proto-oncogenes: normal genes that promote cell growth. When mutated or overexpressed, they become oncogenes — driving uncontrolled proliferation.
• Carcinogens: agents that cause cancer — chemicals (tobacco tar, benzene), physical agents (UV, X-rays) and biological agents (HPV, Epstein-Barr virus).
• Benign tumours: remain localised, enclosed by a capsule, do not invade other tissues.
• Malignant tumours: invade surrounding tissues and spread via blood/lymph to distant sites (metastasis).

• Detection methods: biopsy (tissue sample under microscope), imaging (X-ray, MRI, CT, PET scans), blood tests for tumour markers (PSA, CEA), molecular detection of oncogene mutations.
• Treatment: surgery (remove tumour), radiotherapy (targeted radiation), chemotherapy (anti-cancer drugs — inhibit rapidly dividing cells). Modern approaches include immunotherapy and targeted therapy.

AIDS — HIV and Prevention

• Transmission routes: sexual contact (most common), transfusion of infected blood, sharing contaminated needles, mother to child (vertical — during pregnancy, delivery or breastfeeding). NOT spread by casual contact, food, water or mosquito bites.
• Prevention: safe sex (condoms), screening of blood and organ donations, use of disposable/sterile needles, avoiding sharing personal items (razors, toothbrushes).
• Treatment: HAART (Highly Active Antiretroviral Therapy) — a cocktail of drugs including reverse transcriptase inhibitors, protease inhibitors and integrase inhibitors. It suppresses viral replication but does not cure the disease.
• No vaccine currently available. Early detection through ELISA and Western blot tests allows timely HAART initiation.

Drugs and Alcohol Abuse

Substance abuse refers to the harmful use of psychoactive drugs and alcohol, leading to physical dependence, tolerance and withdrawal symptoms. Major categories relevant to NCERT:

• Opioids: derived from the opium poppy (Papaver somniferum). Include morphine, heroin (diacetylmorphine) and codeine. Act on opioid receptors in the brain, producing analgesia, euphoria and respiratory depression. Highly addictive; heroin is injected intravenously.
• Cannabinoids: active principle in Cannabis sativa (bhang, ganja, charas). Bind to cannabinoid receptors in the brain. Effects: altered perception, relaxation, impaired coordination.
• Cocaine: extracted from Erythroxylum coca. A powerful CNS stimulant. Causes euphoria, increased energy and alertness. Highly addictive; can cause cardiac arrest.
• Hallucinogens (psychedelics): LSD (from ergot fungus Claviceps purpurea), mescaline (from cactus). Alter perception, mood and thought. Cause visual and auditory hallucinations.
• Tobacco: contains nicotine — a stimulant and highly addictive alkaloid. Increases heart rate and blood pressure. Long-term use causes lung cancer, bronchitis, emphysema, cardiovascular disease. Also contains tar and carcinogenic hydrocarbons.

Solved Examples

Example: A patient shows recurrent episodes of fever with chills every 48 hours. Blood smear reveals ring-stage parasites in RBCs. Identify the most likely Plasmodium species, the vector and one diagnostic test.

Solution: Fever every 48 hours (tertian periodicity) with ring-stage parasites in RBCs points to Plasmodium vivax (benign tertian malaria). The vector is the female Anopheles mosquito. Diagnostic methods include blood smear examination under a microscope (stained with Giemsa stain) to detect the parasite, or the rapid diagnostic test (RDT) based on HRP2 antigen.

Example: Distinguish between active immunity and passive immunity with one example each.

Solution: Active immunity involves the body producing its own antibodies after exposure to an antigen, providing long-lasting memory — example: recovering from chickenpox gives lifelong immunity. Passive immunity involves receiving ready-made antibodies from another source, providing immediate but temporary protection — example: a mother transfers IgA antibodies to her infant through breast milk.