Viral, Bacterial and Protozoan Diseases

Why Diseases Are a Guaranteed-Marks Topic

A disease is any condition that disturbs the normal working of the body. Diseases that are caused by pathogens (disease-causing microbes) and can pass from one person to another are called communicable or infectious diseases. The microbes responsible fall into a few families — viruses, bacteria, protozoa, fungi and worms.

The CDS & OTA paper almost always carries one or two questions of the form: “Which of the following is a viral disease?” or “Malaria is spread by which mosquito?” These are pure factual-recall marks. If you memorise the cause and the carrier for each common disease, you simply cannot get them wrong. Unlike physics numericals or chemistry equations, this topic needs no calculation at all — only organised memory. That makes it one of the safest places on the paper to pick up full marks, provided you revise the lists carefully.

The microbes that cause infectious disease are together called germs. They enter the body through air we breathe, water and food we swallow, cuts in the skin, or the bites of insects. Understanding the route of entry helps you answer prevention questions too: an air-borne disease is fought with masks and isolation, a water-borne one with clean water and sanitation, and an insect-borne one by destroying the vector.

Knowing Your Microbes

Before listing diseases, fix the difference between the three pathogen groups, because the exam loves to test whether you can tell a virus from a bacterium.

• Viruses — the smallest pathogens, visible only under an electron microscope. They are not fully living: they can multiply only inside a host cell. Antibiotics do not work against them; we rely on vaccines.
• Bacteria — single-celled organisms larger than viruses, with a cell wall but no true nucleus (prokaryotes). Most bacterial diseases can be cured with antibiotics.
• Protozoa — single-celled animal-like organisms (eukaryotes), bigger than bacteria. They cause diseases such as malaria and amoebic dysentery.

A simple way to picture the size order is: virus < bacterium < protozoan. Viruses are so small that hundreds could sit on a single bacterium, while a protozoan such as the malaria parasite is larger still. Size is not just trivia — it explains why viruses slip into our cells so easily and why ordinary medicines that target a bacterial cell wall fail completely against them.

Two more groups are worth a passing mention because the exam may list them as distractors. Fungi cause skin diseases such as ringworm and athlete’s foot, while helminths (worms) such as the roundworm and tapeworm cause intestinal infections. These are not viral, bacterial or protozoan, so watch for them in “odd one out” questions.

Common Viral Diseases

Viral diseases are caused by viruses and are usually prevented by vaccination rather than cured by medicine. Here are the high-yield ones for CDS.

• Common cold & Influenza (flu) — spread by droplets through air; affect the respiratory tract.
• Polio (Poliomyelitis) — affects the nervous system, can cause paralysis; spread by contaminated water/food; prevented by the OPV (oral polio vaccine) and Salk vaccine.
• Measles, Mumps, Chickenpox, Smallpox — spread by contact/droplets. Smallpox (caused by Variola) was the first disease globally eradicated.
• Dengue and Chikungunya — spread by the Aedes mosquito, which bites in the daytime. Dengue causes high fever and a fall in platelet count.
• Rabies (Hydrophobia) — caused by a virus transmitted by the bite of an infected dog; affects the nervous system; almost always fatal once symptoms appear.
• Hepatitis B — a liver infection spread by blood and body fluids; vaccine available.
• AIDS — caused by HIV, which attacks the immune system; spread by blood, unsafe sex and from mother to child. No vaccine yet.
• COVID-19 — caused by the SARS-CoV-2 coronavirus; spread by respiratory droplets.

Common Bacterial Diseases

Bacterial diseases are caused by bacteria and most are treatable with antibiotics. Several also have effective vaccines.

• Tuberculosis (TB) — caused by Mycobacterium tuberculosis; affects the lungs; spread by droplets (coughing); prevented by the BCG vaccine.
• Cholera — caused by Vibrio cholerae; affects the intestine; spread by contaminated water and food; causes severe watery diarrhoea and dehydration.
• Typhoid — caused by Salmonella typhi; affects the intestine; spread by contaminated water/food; tested by the Widal test.
• Tetanus (Lockjaw) — caused by Clostridium tetani entering through wounds; prevented by the tetanus toxoid (ATT/DPT) injection.
• Diphtheria, Whooping cough (Pertussis) — respiratory diseases prevented by the DPT vaccine.
• Plague — caused by Yersinia pestis; spread by the rat flea.
• Leprosy (Hansen’s disease) — caused by Mycobacterium leprae; affects skin and nerves.
• Pneumonia — infection of the lungs; Tetanus, Typhoid, TB, Cholera are the most-asked bacterial diseases.

Common Protozoan Diseases

Protozoan diseases are caused by single-celled animal-like organisms and several are spread by insect vectors.

• Malaria — caused by the protozoan Plasmodium; spread by the bite of the female Anopheles mosquito; causes cyclic fever with chills; affects the liver and red blood cells.
• Amoebic dysentery (Amoebiasis) — caused by Entamoeba histolytica; affects the large intestine; spread by contaminated food/water; causes loose stools with blood and mucus.
• Kala-azar (Visceral Leishmaniasis) — caused by Leishmania; spread by the bite of the sandfly.
• Sleeping sickness — caused by Trypanosoma; spread by the tsetse fly (mainly in Africa).
• Giardiasis — caused by Giardia; affects the intestine.

In malaria the parasite has two homes: it multiplies inside the human liver and red blood cells, then is picked up by a mosquito when it bites. The bursting of infected red cells releases the parasite in waves, which is why malarial fever comes in regular cycles with chills and sweating. Famously, Sir Ronald Ross discovered the role of the mosquito in spreading malaria while working in India, and won a Nobel Prize for it.

Carriers and Vectors at a Glance

The examiner loves to test which insect or animal carries which disease. Burn this short list into memory.

• Female Anopheles mosquito → Malaria
• Aedes mosquito → Dengue, Chikungunya, Yellow fever, Zika
• Culex mosquito → Filariasis (elephantiasis), Japanese encephalitis
• Housefly → Cholera, Typhoid, Dysentery (mechanical carrier)
• Sandfly → Kala-azar
• Tsetse fly → Sleeping sickness
• Rat flea → Plague
• Dog bite → Rabies

Vaccines and Prevention

A vaccine contains a weakened or dead form of a pathogen (or its parts). When injected, it trains the immune system to make antibodies in advance, so the body is ready if the real microbe attacks. This is the principle of immunisation, first demonstrated by Edward Jenner for smallpox in 1796 when he used cowpox material to protect against the deadly smallpox virus — the word “vaccine” itself comes from vacca, the Latin for cow.

There are two broad kinds of immunity worth knowing. Active immunity develops when the body makes its own antibodies, either after an infection or after a vaccine; it is slow to appear but long-lasting. Passive immunity is borrowed — ready-made antibodies are given directly, as in an anti-tetanus or anti-rabies injection after an injury; it acts at once but fades quickly. The exam occasionally asks which type a particular injection provides.

Vaccine-to-disease quick list

• BCG → Tuberculosis
• OPV / IPV → Polio
• DPT → Diphtheria, Pertussis (whooping cough), Tetanus
• MMR → Measles, Mumps, Rubella
• Hepatitis B vaccine → Hepatitis B

Infectious vs Non-Infectious Diseases

Not every disease is caused by a microbe. The exam sometimes slips in a non-communicable disease to test whether you can tell the difference.

• Communicable / infectious — caused by pathogens and spread between people (TB, cholera, COVID-19, malaria).
• Non-communicable — do not spread; caused by lifestyle, genetics or deficiency (diabetes, cancer, hypertension).
• Deficiency diseases — caused by lack of a nutrient: Scurvy (lack of vitamin C), Rickets (vitamin D), Beriberi (vitamin B₁), Goitre (iodine), Night blindness (vitamin A), Anaemia (iron).

Worked Example: Classifying a Disease

Let us apply the rules to a typical sorting question.

Notice the trap: malaria and dengue are both mosquito-borne, so candidates often group them together. But the cause differs — malaria is a protozoan disease, dengue is viral. Always classify by the pathogen, not by the carrier.

Mistakes That Cost Easy Marks

These slip-ups appear again and again in CDS answer keys. Avoid them and you protect guaranteed marks.

• Confusing malaria (protozoan) with dengue (viral) just because both spread via mosquitoes.
• Mixing up the mosquitoes: Anopheles = malaria, Aedes = dengue, Culex = filaria.
• Thinking antibiotics cure viral diseases — they do not.
• Forgetting that male mosquitoes do not bite; only the female mosquito transmits disease.
• Calling deficiency diseases like scurvy or goitre “infectious” — they are not.

Previous-Year Style Question

Practice with a question framed the way the CDS examiner writes it.

Quick Revision