7.1 Common Infectious Diseases in Humans
| Disease | Pathogen | Transmission | Key Symptoms |
|---|
| Typhoid | Salmonella typhi (bacterium) | Contaminated food/water | Sustained high fever 39–40°C, intestinal perforation. Confirmed by Widal test. |
| Pneumonia | Streptococcus pneumoniae, Haemophilus influenzae | Inhaling droplets; sharing utensils | Fever, chills, cough; alveoli fill with fluid; lips/nails turn gray/bluish. |
| Common Cold | Rhino viruses | Contaminated objects, droplets | Nasal congestion, sore throat, cough; lasts 3–7 days. Infects nose/respiratory passage only, NOT lungs. |
| Malaria | Plasmodium spp. (protozoan); P. falciparum = most deadly | Female Anopheles mosquito (vector) | Chill and high fever every 3–4 days due to haemozoin release. |
| Amoebiasis | Entamoeba histolytica | Contaminated food/water; houseflies as mechanical carriers | Constipation, abdominal pain, stools with mucous and blood. |
| Ascariasis | Ascaris (roundworm) | Contaminated soil/water/plants | Internal bleeding, muscle pain, fever, anaemia, intestinal blockage. |
| Filariasis (Elephantiasis) | Wuchereria bancrofti / W. malayi | Female mosquito bite | Chronic inflammation of lymphatic vessels of lower limbs; gross deformity. |
| Ringworm | Fungi: Microsporum, Trichophyton, Epidermophyton | Soil; shared towels/combs | Dry, scaly lesions on skin, nails, scalp; intense itching. |
Typhoid Mary: Mary Mallon, a cook who was a typhoid carrier — spread typhoid for years through food without showing symptoms herself.
Malaria parasite needs two hosts: human and female Anopheles mosquito. Sporozoites enter via mosquito bite → multiply in liver → attack RBCs → haemozoin released.
7.2 Immunity — Innate and Acquired
Innate Immunity (Non-specific, present at birth)
| Barrier | Examples |
|---|
| Physical | Skin; mucus coating of epithelium lining respiratory, GI, urogenital tracts |
| Physiological | Acid in stomach; saliva; tears — prevent microbial growth |
| Cellular | PMNL-neutrophils, monocytes, natural killer cells (lymphocytes), macrophages — phagocytose microbes |
| Cytokine | Interferons — secreted by virus-infected cells; protect non-infected cells from viral infection |
Acquired Immunity (Specific, has memory)
- Primary response: first encounter with pathogen — low intensity.
- Secondary (anamnestic) response: subsequent encounter — highly intensified, due to memory B & T cells.
- B-lymphocytes: produce antibodies (humoral immune response).
- T-lymphocytes: cell-mediated immunity (CMI) — responsible for graft rejection.
- Antibody structure: 4 peptide chains — 2 light + 2 heavy (H₂L₂). Types: IgA, IgM, IgE, IgG.
Active vs Passive Immunity
- Active: body produces antibodies itself (slow; via vaccination or natural infection). Memory generated.
- Passive: ready-made antibodies given (quick but no memory). Examples: colostrum (IgA for newborn), antitoxin for snake bite, tetanus antitoxin.
Colostrum = yellowish fluid secreted by mother in early lactation → rich in IgA antibodies for the newborn.
Allergy
Exaggerated immune response to environmental antigens (allergens). Antibodies produced: IgE type. Mast cells release histamine and serotonin → symptoms: sneezing, watery eyes, running nose, breathing difficulty. Treatment: anti-histamines, adrenalin, steroids.
Auto-immunity
Body attacks its own cells. Example: Rheumatoid arthritis.
Lymphoid Organs
- Primary: bone marrow (all blood cells including lymphocytes), thymus (T-lymphocyte maturation). Thymus is large at birth, reduces to very small size by puberty.
- Secondary: spleen (filter blood, reservoir of erythrocytes), lymph nodes (trap microorganisms from lymph), tonsils, Peyer's patches, appendix.
- MALT (Mucosa-Associated Lymphoid Tissue): ~50% of lymphoid tissue in body, lining respiratory/digestive/urogenital tracts.
7.3 AIDS
AIDS = Acquired Immuno Deficiency Syndrome. First reported in 1981. Killed >25 million people worldwide.
Caused by HIV (Human Immunodeficiency Virus) — a retrovirus with RNA genome enclosed in an envelope.
Transmission routes
- Sexual contact with infected person.
- Transfusion of contaminated blood/blood products.
- Sharing infected needles (IV drug abusers).
- Infected mother to child through placenta.
Mechanism (How HIV destroys immunity)
HIV enters macrophages→
RNA → viral DNA (reverse transcriptase)→
Viral DNA integrates into host DNA→
Produces virus particles (HIV factory)→
HIV attacks helper T-lymphocytes (TH)→
Progressive decrease in helper T-cells→
Immune deficiency → opportunistic infections → death
- Incubation period: few months to many years (usually 5–10 years).
- Opportunistic infections: Mycobacterium, viruses, fungi, Toxoplasma.
- Diagnosis: ELISA (most widely used). Treatment: anti-retroviral drugs (partial effect only — prolongs life, cannot prevent death).
HIV does NOT spread by touch or physical contact — only through body fluids. HIV/AIDS infected persons should NOT be isolated from family/society.
7.4 Cancer
Cancer arises when contact inhibition is lost — cells divide uncontrollably forming tumors.
Types of tumors
- Benign: confined to original location, does not spread. Causes little damage.
- Malignant: proliferating neoplastic cells that invade surrounding tissue and spread via blood to new sites = metastasis. Most feared property.
Causes (Carcinogens)
- Physical: ionising radiation (X-rays, gamma rays), non-ionising (UV).
- Chemical: chemicals in tobacco smoke — major cause of lung cancer.
- Biological: oncogenic viruses (carry viral oncogenes). Proto-oncogenes (c-onc) in normal cells can become oncogenes under certain conditions.
Detection
- Biopsy (tissue staining + microscopy).
- Radiography (X-rays), CT scan, MRI (for internal cancers).
- Antibodies against cancer-specific antigens.
- Molecular biology for gene detection.
Treatment
- Surgery, radiation therapy (radiotherapy), chemotherapy.
- Immunotherapy: α-interferon (biological response modifier) activates immune system to destroy tumor.
7.5 Drugs and Alcohol Abuse
| Drug Class | Source | Example | Effect |
|---|
| Opioids | Latex of poppy plant Papaver somniferum | Heroin (smack / diacetylmorphine) | Binds opioid receptors in CNS & GIT; depressant — slows body functions; taken by snorting/injection. |
| Cannabinoids | Inflorescences of Cannabis sativa | Marijuana, hashish, charas, ganja | Interact with cannabinoid receptors in brain; effects on cardiovascular system; taken by inhalation/oral. |
| Coca alkaloids | Erythroxylum coca (South America) | Cocaine (coke/crack) | Interferes with dopamine transport; stimulates CNS; euphoria, increased energy; excessive dose → hallucinations. |
Effects of drug/alcohol abuse
- Immediate: reckless behaviour, violence, coma, death (respiratory/heart failure).
- IV drug use → HIV, Hepatitis B (via shared needles).
- Chronic: nervous system damage, liver cirrhosis.
- Anabolic steroids (sports doping): females — masculinisation, abnormal menstrual cycles; males — testicular atrophy, decreased sperm count, breast enlargement (gynaecomastia).
Nicotine (tobacco): stimulates adrenal gland to release adrenaline and nor-adrenaline → increased blood pressure and heart rate. Associated with lung, bladder, throat cancers.