Health is defined as a state of complete physical, mental, and social well-being (not merely the absence of disease). Common Diseases in Humans: Pathogens are disease-causing organisms.

• • Bacterial Diseases:

    • Typhoid: Caused by Salmonella typhi. Enters small intestine through contaminated food/water. Symptoms: sustained high fever (39°–40°C), weakness, stomach pain, constipation, headache, loss of appetite. Confirmed by Widal test.

    • Pneumonia: Caused by Streptococcus pneumoniae and Haemophilus influenzae. Infects alveoli (fluid-filled). Symptoms: fever, chills, cough, headache; in severe cases, lips and fingernails turn gray to bluish.

  • Viral Diseases:

    • Common Cold: Caused by Rhinoviruses. Infects nose and respiratory passage but not the lungs. Lasts 3-7 days.

  • Protozoan Diseases:

    • Malaria: Caused by Plasmodium (P. vivax, P. malariae, P. falciparum). P. falciparum causes the most serious (malignant) malaria.

      • Life Cycle: The malarial parasite enters the human body as sporozoites (infectious form) through the bite of an infected female Anopheles mosquito. Parasites multiply within liver cells, then attack Red Blood Cells (RBCs) resulting in their rupture. The rupture releases a toxic substance, haemozoin, responsible for the chill and high fever recurring every 3-4 days. Sexual stages (gametocytes) develop in RBCs. The mosquito takes these up; fertilization and development take place in the mosquito’s gut. Mature sporozoites migrate to the mosquito’s salivary glands.

Amoebiasis (Amoebic Dysentery): Caused by "Entamoeba histolytica" in the large intestine. 

Houseflies act as mechanical carriers.

 Symptoms: constipation, abdominal pain, stools with excess mucous and blood clots.

• Helminthic Diseases:

  • Ascariasis: Caused by Ascaris (common roundworm). Internal bleeding, muscular pain, fever, anemia, blockage of intestinal passage.

  • Elephantiasis (Filariasis): Caused by Wuchereria (W. bancrofti and W. malayi). Chronic inflammation of lymphatic vessels, usually of lower limbs.

• Fungal Diseases:

  • Ringworm: Caused by Microsporum, Trichophyton, and Epidermophyton. Dry, scaly lesions on skin, nails, scalp accompanied by intense itching. Heat and moisture help fungi grow.

Immunity:

• Innate Immunity: Non-specific defense present at birth. Four barriers:

  1. Physical: Skin, Mucus coating (respiratory, gastrointestinal, urogenital tracts).

  2. Physiological: Acid in stomach, saliva in mouth, tears from eyes.

  3. Cellular: Polymorphonuclear leukocytes (PMNL-neutrophils), Monocytes, Natural Killer (NK) cells, Macrophages (phagocytosis).

  4. Cytokine: Virus-infected cells secrete proteins called Interferons which protect non-infected cells from further viral infection.

• Acquired Immunity: Pathogen-specific, characterized by memory. Primary response is low intensity; Secondary (anamnestic) response is highly intensified. Carried out by:

  • B-lymphocytes: Produce an army of proteins called antibodies into the blood (Humoral Immune Response).

  • T-lymphocytes: Help B-cells produce antibodies and mediate Cell-mediated Immunity (CMI). CMI is responsible for Graft Rejection.

  • Antibody Structure: Represented as H2L2 (two light chains, two heavy chains). Examples: IgA, IgM, IgE, IgG. IgA is present in colostrum (yellowish fluid produced during initial days of lactation).

• Active vs Passive Immunity: Active is slow (body produces own antibodies). Passive is ready-made antibodies given (e.g., Colostrum, Anti-tetanus serum).

• Allergy: Exaggerated response of the immune system to antigens (allergens). Involves IgE antibodies. Release of histamine and serotonin from mast cells causes symptoms. Treated with antihistamine, adrenaline, and steroids.

• Autoimmunity: Body attacks self-cells (e.g., Rheumatoid arthritis).

• Lymphoid Organs:

  • Primary: Bone marrow (all blood cells including lymphocytes produced) and Thymus (T-lymphocytes mature; degenerates with age).

  • Secondary: Spleen (filter of blood, reservoir of erythrocytes), Lymph nodes, Tonsils, Peyer’s patches (small intestine). MALT (Mucosa-associated lymphoid tissue) constitutes 50% of lymphoid tissue.

AIDS (Acquired Immuno Deficiency Syndrome): Caused by HIV (Human Immunodeficiency Virus), a retrovirus (RNA genome). Transmission via sexual contact, contaminated blood, shared needles, or placenta.

• • Mechanism: Virus enters macrophages (acts as HIV factory). RNA genome replicates to form viral DNA via reverse transcriptase. Virus enters Helper T-lymphocytes (TH), replicates, and destroys them. Decrease in TH count leads to opportunistic infections (Mycobacterium, viruses, fungi).

• Diagnosis: ELISA (Enzyme Linked Immuno-Sorbent Assay). Treatment with anti-retroviral drugs is only partially effective.

Cancer: Normal cells show contact inhibition; cancer cells lose this property.

• Tumors: Benign (confined to original location) and Malignant (mass of proliferating neoplastic cells). Malignant cells starve normal cells and show Metastasis (spread to distant sites via blood).

• Causes: Carcinogens (physical: X-rays, UV rays; chemical: tobacco smoke; biological: oncogenic viruses). Cellular oncogenes (c-onc) in normal cells can be activated to oncogenes.

• Detection: Biopsy, Histopathological studies, Radiography (X-rays), CT (Computed Tomography – 3D image), MRI (Magnetic Resonance Imaging – uses strong magnetic fields).

• Treatment: Surgery, Radiation therapy, Chemotherapy (side effects: hair loss, anemia). $\alpha$-interferons (biological response modifiers) activate the immune system to destroy the tumor.

Drugs and Alcohol Abuse:

• Opioids: Bind to specific opioid receptors in CNS and gastrointestinal tract. Heroin (Smack) is chemically diacetylmorphine (white, odourless, bitter crystalline compound), obtained by acetylation of morphine from latex of Poppy plant (Papaver somniferum). It is a depressant and slows down body functions.

• Cannabinoids: Interact with cannabinoid receptors in the brain. Obtained from inflorescences of Cannabis sativa. Includes Marijuana, hashish, charas, ganja. Taken by inhalation/oral ingestion. Affects cardiovascular system.

• Coca alkaloid: Cocaine (Coke/Crack) from Erythroxylum coca (South America). Interferes with transport of neuro-transmitter dopamine. Potent stimulating action on CNS, producing euphoria and increased energy.

• Hallucinogens: Atropa belladonna, Datura.

• Tobacco: Contains Nicotine (stimulates adrenal gland to release adrenaline/noradrenaline, raising BP and heart rate). Smoking is associated with lung cancer, bronchitis, emphysema, coronary heart disease.

• Adolescence and Drug Abuse: Addiction is a psychological attachment to certain effects (euphoria). Repeated use increases tolerance level of receptors. Dependence is the tendency of the body to manifest a characteristic and unpleasant withdrawal syndrome if the dose is abruptly discontinued. Chronic use leads to Cirrhosis of the liver.