Viral, Bacterial and Protozoan Diseases
~8 min read
- Viral: Dengue, Influenza, Hepatitis, HIV-AIDS, COVID-19. No antibiotics work. Vaccines are key.
- Bacterial: Tuberculosis, Cholera, Typhoid, Tetanus. Antibiotics effective.
- Protozoan: Malaria (Plasmodium), Amoebic dysentery, Kala-azar (Leishmania).
Communicable diseases are a steady CDS/OTA theme — especially modes of spread and vaccines.
Viral Diseases
| Disease | Virus | Spread by |
|---|---|---|
| Dengue | Flavi-ribo virus (Dengue virus) | Aedes aegypti mosquito (day-biter) |
| AIDS | HIV (destroys lymphocytes — CD4 T cells) | Contaminated blood transfusion, sexual contact, mother-to-child |
| Influenza | Influenza virus | Droplets in air |
| Hepatitis B/C | HBV/HCV | Blood, sexual contact |
| Smallpox | Variola virus | Eradicated 1980 by vaccination |
| COVID-19 | SARS-CoV-2 | Droplets, aerosols |
HIV transmission: through contaminated blood and blood products, sexual contact, shared needles, mother-to-child. NOT through casual contact, food/water, air, or shaking hands.
Bacterial Diseases
- Tuberculosis (TB): Mycobacterium tuberculosis. Spreads through droplets in air when an infected person coughs or sneezes. Crowded places raise risk.
- Cholera: Vibrio cholerae. Spreads via contaminated water and food.
- Typhoid: Salmonella typhi. Faeco-oral via contaminated water.
- Tetanus: Clostridium tetani. Enters through wounds; produces neurotoxin.
- Treatment: antibiotics (penicillin, isoniazid, ciprofloxacin etc.) work because they target bacterial cell walls or ribosomes that animal cells lack.
Protozoan and Other Parasitic Diseases
- Malaria: caused by Plasmodium protozoan. Spread by female Anopheles mosquito.
- Amoebic dysentery: Entamoeba histolytica. Contaminated water/food.
- Kala-azar (visceral leishmaniasis): Leishmania. Spread by sandfly.
- Filariasis (elephantiasis): Wuchereria bancrofti (a nematode worm). Spread by Culex mosquito.
- Elephantiasis is endemic, not epidemic — it persists at a steady level in affected regions rather than spreading in sudden outbreaks.
Major Vaccines
| Vaccine | Type |
|---|---|
| Covaxin | Inactivated whole-virus (Bharat Biotech) |
| Covishield | Viral vector (adenovirus carrying spike gene); NOT mRNA |
| Sputnik V | Viral vector (two different adenoviruses) |
| Pfizer-BioNTech, Moderna | mRNA vaccines |
| BCG | Live attenuated (against TB) |
CDS/OTA PYQ Examples
Q: Which one of the following diseases in humans can spread through air?
(a) Dengue (b) Tuberculosis (c) HIV-AIDS (d) Goitre
Answer: (b) Tuberculosis — droplets. [CDS-I 2015]
Q: HIV virus weakens immunity because it destroys:
(a) Mast cells (b) Platelets (c) Erythrocytes (d) Lymphocytes
Answer: (d) Lymphocytes — particularly CD4 T cells. [CDS-II 2016]
Q: HIV transmission generally occurs through:
(a) Contaminated food and water (b) Transfusion of contaminated blood (c) Polluted air (d) Hand-shaking
Answer: (b) Contaminated blood and blood products (and sexual contact, mother-to-child). [CDS-I 2017]
Q: Which is NOT an epidemic disease?
(a) Cholera (b) Malaria (c) Smallpox (d) Elephantiasis
Answer: (d) Elephantiasis — endemic, not epidemic. [CDS-I 2020]
Q: Causal organism of dengue fever belongs to:
(a) Flavi-ribo virus (b) Adenovirus (c) Vaccinia virus (d) Nipah virus
Answer: (a) Flavi-ribo virus (Dengue virus). [CDS-I 2024]
Q: Covaxin – Inactivated; Covishield – mRNA; Sputnik V – Viral vector. Which pairs are correctly matched?
(a) 1 only (b) 2 and 3 only (c) 1 and 3 only (d) 1, 2 and 3
Answer: (c) 1 and 3 — Covishield is viral vector, NOT mRNA. [CDS-I 2022]
Drill Viral, Bacterial and Protozoan Diseases for CDS/OTA
CDS/OTA-pattern items on Viral, Bacterial and Protozoan Diseases with answer keys and explanations.
Start Free Mock TestFrequently Asked Questions
Why don't antibiotics work against viruses?
Viruses lack cell walls, ribosomes and metabolic enzymes that antibiotics target. They hijack host-cell machinery. Antiviral drugs target specific viral enzymes or entry receptors instead.
What is the difference between epidemic and endemic?
Endemic = constantly present at a base level in a region (malaria in parts of India). Epidemic = sudden outbreak above expected levels in a region (cholera in a flood). Pandemic = epidemic across countries/continents (COVID-19).
How did smallpox become the only eradicated human disease?
Aggressive ring-vaccination, no animal reservoir, easily recognised symptoms and a highly effective vaccine. WHO declared eradication in 1980 after the last natural case in 1977.