1. Bacterial Diseases
| Disease | Causative Organism | Transmission | Key Symptoms & Pathology | Treatment/Prevention |
|---|---|---|---|---|
| Typhoid (Enteric fever) |
Salmonella typhi | Contaminated food and water (faecal-oral route) | Sustained high fever (39–40°C), abdominal pain, constipation then diarrhoea, rose-coloured spots on abdomen, headache, intestinal perforation in severe cases | Antibiotics (chloramphenicol, ciprofloxacin); typhoid vaccine; safe water |
| Pneumonia | Streptococcus pneumoniae; Haemophilus influenzae | Droplets from infected person; inhaling air near infected person | Fever, chills, cough with mucus/blood-stained sputum, breathlessness; alveoli fill with fluid — impairs gas exchange. Lips and fingernails turn greyish-blue (cyanosis) | Antibiotics; pneumococcal vaccine |
| Tuberculosis (TB) | Mycobacterium tuberculosis | Airborne droplets from infected person (coughing, sneezing) | Persistent cough (may have blood-stained sputum), low-grade fever, night sweats, weight loss, fatigue; primarily affects lungs (pulmonary TB); can affect other organs (extra-pulmonary) | DOTS therapy (multi-drug: rifampicin, isoniazid, pyrazinamide, ethambutol); BCG vaccine |
| Cholera | Vibrio cholerae | Contaminated water and food; faecal-oral route | Profuse watery diarrhoea ("rice water stools"), vomiting, severe dehydration, muscle cramps; cholera toxin causes massive Cl⁻ secretion into gut lumen | ORS (oral rehydration salts); antibiotics; safe water; cholera vaccine |
| Tetanus (Lockjaw) |
Clostridium tetani | Wound infection (soil-contaminated wounds, rusty nails); spores germinate in anaerobic conditions | Tetanospasmin toxin blocks inhibitory neurotransmitters → sustained muscle spasms; lockjaw (trismus), opisthotonos (back arching), risus sardonicus (sardonic grin) | Tetanus toxoid vaccine (TT); antitoxin injection; wound cleaning |
Widal Test — Diagnosis of Typhoid
The Widal test detects agglutinating antibodies against Salmonella typhi O and H antigens in the patient's serum. A rising titre of antibodies is diagnostic. It is a serological test — not a direct culture. Mary Mallon ("Typhoid Mary") is the famous historical example of an asymptomatic carrier of typhoid.
2. Viral Diseases
| Disease | Causative Virus | Transmission | Key Symptoms & Pathology | Prevention |
|---|---|---|---|---|
| Common Cold | Rhinovirus (most common; >100 serotypes) | Droplets; contact with contaminated surfaces | Nasal congestion, runny nose, sore throat, mild fever, headache, cough; affects nose and respiratory passage (not lungs) | No specific vaccine (too many serotypes); symptomatic treatment; hygiene |
| Influenza (Flu) |
Influenza virus (A, B, C) | Droplets; airborne | High fever, severe myalgia, headache, fatigue, cough; can lead to pneumonia in vulnerable populations | Annual influenza vaccine (updated for circulating strains); antivirals (oseltamivir) |
| Mumps | Mumps virus (Paramyxovirus) | Droplets; saliva | Painful swelling of parotid salivary glands (parotitis); fever; can cause orchitis (testicular inflammation) in post-pubertal males → infertility | MMR vaccine (Measles-Mumps-Rubella) |
| Measles (Rubeola) |
Measles virus (Paramyxovirus) | Highly contagious; airborne droplets | High fever, cough, runny nose; Koplik's spots (white spots inside mouth — pathognomonic); red maculopapular rash spreading from head downward; can cause encephalitis, blindness | MMR vaccine |
| Chickenpox (Varicella) |
Varicella-Zoster Virus (VZV); Herpesvirus | Highly contagious; airborne; direct contact with vesicles | Mild fever; itchy rash progressing from macules → papules → vesicles → pustules → crusts; starts on trunk, spreads outward; virus remains latent in dorsal root ganglia → can reactivate as shingles (herpes zoster) | Varicella vaccine; antivirals (acyclovir) for severe cases |
| Hepatitis B | Hepatitis B Virus (HBV); Hepadnavirus | Blood, sexual contact, mother to child (vertical transmission); NOT by casual contact | Jaundice (yellowing of skin/eyes), fatigue, nausea, dark urine, abdominal pain; chronic infection → cirrhosis, hepatocellular carcinoma | HBsAg vaccine (recombinant); safe sex; safe needles |
| Dengue | Dengue virus (Flavivirus; 4 serotypes) | Bite of Aedes aegypti mosquito (day-biting) | "Breakbone fever" — severe joint and muscle pain, high fever, rash, thrombocytopenia (low platelets); severe form: Dengue Haemorrhagic Fever (DHF) → internal bleeding | No specific antiviral; mosquito control; supportive care |
3. Protozoan Diseases
| Disease | Causative Organism | Vector/Transmission | Key Pathology | Treatment |
|---|---|---|---|---|
| Malaria | Plasmodium vivax (benign tertian), P. falciparum (malignant/cerebral — most dangerous), P. malariae, P. ovale | Bite of female Anopheles mosquito (night-biting) | Sporozoites enter liver → merozoites rupture RBCs → cyclical fever with chills (every 48 or 72 hours); anaemia; splenomegaly. P. falciparum → cerebral malaria, blackwater fever (haemoglobinuria) | Chloroquine (resistance rising); artemisinin-based combination therapy (ACT); mosquito nets; DDT |
| Amoebiasis (Amoebic dysentery) |
Entamoeba histolytica | Faecal-oral route; contaminated water/food; houseflies as mechanical vectors | Abdominal pain, bloody and mucus-containing stools (dysentery); trophozoites invade large intestine mucosa; can form liver abscess | Metronidazole; safe water; sanitation |
| Sleeping Sickness | Trypanosoma brucei | Bite of Tsetse fly (Glossina species) | Fever, headache, joint pain; later CNS involvement — confusion, disrupted sleep-wake cycle, coma; prevalent in sub-Saharan Africa | Suramin, melarsoprol; tsetse fly control |
| Kala-azar (Visceral leishmaniasis) |
Leishmania donovani | Bite of female sandfly (Phlebotomus) | Irregular fever, weight loss, enlargement of spleen and liver (hepatosplenomegaly), anaemia, darkening of skin (kala = black; azar = fever in Hindi) | Sodium stibogluconate; miltefosine; sandfly control |
Life Cycle of Plasmodium (Malaria) — Key Points
Plasmodium has a complex two-host life cycle:
- Sexual reproduction occurs in the female Anopheles mosquito (definitive host).
- Asexual reproduction (schizogony) occurs in humans (intermediate host).
- Infective stage to human: Sporozoites (in mosquito salivary gland) — injected during mosquito bite.
- Infective stage to mosquito: Gametocytes (in human blood) — taken up when mosquito feeds.
- Liver stage (exo-erythrocytic schizogony): Sporozoites → liver cells → merozoites.
- Blood stage (erythrocytic schizogony): Merozoites infect RBCs → trophozoites → schizonts → RBC ruptures releasing merozoites → cyclical fever.
- Fever cycle: P. vivax and P. ovale — every 48 hours (tertian); P. malariae — every 72 hours (quartan); P. falciparum — every 36–48 hours (malignant tertian).
4. Helminthic (Worm) Diseases
| Disease | Causative Organism | Transmission | Key Pathology | Treatment |
|---|---|---|---|---|
| Ascariasis | Ascaris lumbricoides (roundworm) |
Ingestion of embryonated eggs from soil-contaminated food/water | Internal bleeding, muscular pain, fever, anaemia; blockage of intestine/bile duct; larvae migrate through lungs (Löffler's syndrome — eosinophilic pneumonia) | Albendazole, mebendazole; sanitation; handwashing |
| Filariasis (Elephantiasis) |
Wuchereria bancrofti; W. malayi (filarial worms) |
Bite of female Culex mosquito | Adult worms live in lymphatic vessels → block lymph flow → lymphoedema → massive swelling of limbs and genitals (elephantiasis); chronic, disfiguring condition | Diethylcarbamazine (DEC); mass drug administration; mosquito control |
| Enterobiasis (Pinworm infection) |
Enterobius vermicularis (pinworm/threadworm) |
Faecal-oral; eggs ingested; most common helminthic infection in temperate countries | Intense perianal itching (especially at night when female migrates to lay eggs); insomnia, irritability, secondary bacterial infection from scratching | Albendazole, mebendazole; hygiene |
| Taeniasis (Tapeworm infection) |
Taenia solium (pork tapeworm); T. saginata (beef tapeworm) | Ingestion of undercooked pork/beef containing cysticerci (larval cysts) | Adult tapeworm in intestine → absorbs nutrients → weight loss, abdominal pain; cysticercosis: larvae in brain/muscles (neurocysticercosis → seizures, headache) | Praziquantel, niclosamide; cook meat thoroughly |
5. Fungal Diseases
Fungal infections (mycoses) primarily affect the skin, nails, and mucous membranes in immunocompetent individuals. In immunocompromised patients (AIDS, cancer therapy), systemic fungal infections can be life-threatening (opportunistic mycoses).
| Disease | Causative Organism | Site Affected | Treatment |
|---|---|---|---|
| Ringworm (Tinea) | Trichophyton, Microsporum, Epidermophyton | Skin, scalp, nails (not a worm — misnomer) | Antifungal creams (clotrimazole, miconazole); oral griseofulvin for severe cases |
| Candidiasis (thrush) | Candida albicans | Mouth, throat, vagina, skin folds; systemic in immunocompromised | Fluconazole, nystatin |
| Aspergillosis | Aspergillus fumigatus | Lungs (primarily in immunocompromised patients) | Voriconazole, amphotericin B |
Note on Ringworm: Despite its name, ringworm is caused by a fungus, not a worm. It produces ring-shaped, itchy, scaly lesions on the skin. Transmitted by direct contact with infected persons, animals, or contaminated soil. Poor hygiene and sharing of towels/combs are risk factors.
6. Important Vectors and the Diseases They Transmit
| Vector | Species | Diseases Transmitted | Biting Time |
|---|---|---|---|
| Female Anopheles mosquito | Anopheles spp. | Malaria | Night (dusk to dawn) |
| Female Aedes mosquito | Aedes aegypti | Dengue, Chikungunya, Yellow fever, Zika | Day (especially at dawn and dusk) |
| Female Culex mosquito | Culex spp. | Filariasis (Elephantiasis), Japanese encephalitis, West Nile fever | Night |
| Sandfly | Phlebotomus spp. | Kala-azar (Visceral leishmaniasis), Cutaneous leishmaniasis | Night |
| Tsetse fly | Glossina spp. | African Sleeping Sickness (Trypanosomiasis) | Day |
| Housefly | Musca domestica | Typhoid, Cholera, Amoebiasis, Dysentery (mechanical vector — not biological) | Day |
7. Key Diagnostic and Distinguishing Features
| Pathognomonic Sign / Key Feature | Disease |
|---|---|
| Koplik's spots (white spots inside cheeks) | Measles |
| Greyish-blue lips and fingernails (cyanosis) | Pneumonia |
| Rice water stools | Cholera |
| Lockjaw (trismus) + opisthotonus | Tetanus |
| "Breakbone fever" (severe bone/joint pain) | Dengue |
| Blackwater fever (haemoglobinuria) | Malaria (P. falciparum) |
| Parotid gland swelling | Mumps |
| Darkening of skin + hepatosplenomegaly | Kala-azar |
| Elephantiasis (massive limb swelling) | Filariasis (Wuchereria bancrofti) |
| Ring-shaped itchy skin lesions (despite name — caused by fungus) | Ringworm (Tinea) |
