A disease is any condition that disturbs the normal working of the body. Some diseases spread from person to person through tiny living agents called pathogens — bacteria, viruses, protozoa, fungi and worms — while others come from inside, such as deficiency of a vitamin or a faulty gene. For NDA Biology, this is a high-scoring, fact-heavy chapter where the same disease-cause-vector links repeat almost every year.
Why This Topic Matters for NDA
The NDA General Studies paper almost always carries a few Biology questions, and human diseases is one of the most dependable scoring areas. The questions are direct and factual — name the pathogen, name the vector, name the deficient vitamin — so they reward simple, organised memorisation rather than calculation. Unlike Physics or Maths, you do not need a formula here; you need a clear memory of who causes what.
Examiners love clean matching: malaria with Plasmodium and the female Anopheles mosquito, scurvy with vitamin C, or tuberculosis with a bacterium. Once you fix these pairs, you almost never lose the marks. The same handful of diseases — malaria, dengue, tuberculosis, AIDS, polio and the major deficiency diseases — reappear year after year, sometimes worded as “match the column” and sometimes as a single one-line fact.
Because the topic also overlaps with everyday current affairs and health awareness, a strong grip here helps you in the SSB interview and the personal-interaction round too, where candidates are often asked about common diseases and public-health schemes.
Build a single table in your notes: Disease → Pathogen type → Vector/Cause → Affected organ. Revising one table the night before the exam covers most of what is asked here.
What Is a Pathogen?
A pathogen is a disease-causing organism. It is also called a germ. Pathogens are usually microscopic, which means they cannot be seen with the naked eye. They enter the body through air, water, food, direct contact or insect bites, then multiply rapidly and damage tissues or release poisons called toxins. The organism in which a pathogen lives and multiplies is called the host.
The five main groups of pathogens
- Viruses — the smallest, e.g. influenza, polio, COVID-19, AIDS.
- Bacteria — single-celled organisms, e.g. tuberculosis, cholera, typhoid.
- Protozoa — single-celled animals, e.g. malaria, amoebic dysentery.
- Fungi — e.g. ringworm, athlete’s foot.
- Worms (helminths) — e.g. tapeworm, roundworm, filariasis.
Each group has a typical size and behaviour. Viruses are far smaller than bacteria; bacteria are larger but still need a microscope; worms can be several centimetres long and live in the intestines. The size and type of the pathogen decides how the disease spreads and how it is treated.
Viruses are the smallest pathogens and can multiply only inside a living host cell. Antibiotics work against bacteria, not against viruses — a very common NDA trap.
Communicable vs Non-Communicable Diseases
Diseases are broadly divided into two big families. Getting this split clear in your head helps you classify any disease the examiner throws at you.
Communicable (infectious) diseases
These are caused by pathogens and spread from a sick person (or animal) to a healthy one. They travel through air, water, food, direct contact or insect vectors. Examples: tuberculosis, cholera, malaria, dengue, AIDS. Because they spread, an outbreak can quickly grow into an epidemic (affecting many people in an area) or even a pandemic (spreading across countries, as COVID-19 did).
Non-communicable diseases
These do not spread from person to person. They arise from within the body — a missing nutrient, a faulty gene, an organ that stops working, or an unhealthy lifestyle. Examples: scurvy, diabetes, cancer, hypertension, haemophilia. They usually develop slowly and last a long time, which is why many of them are also called chronic diseases.
Modes of spread for communicable diseases
- Air-borne — through coughing and sneezing, e.g. tuberculosis, common cold.
- Water- and food-borne — through dirty water or food, e.g. cholera, typhoid.
- Contact — through touch or body fluids, e.g. AIDS, ringworm.
- Vector-borne — through insects, e.g. malaria, dengue.
Communicable = catchable. If a disease cannot pass from one person to another (like diabetes or scurvy), it is non-communicable.
Common Bacterial Diseases
Bacterial diseases are usually treatable with antibiotics and are often spread through contaminated food and water or droplets.
- Tuberculosis (TB) — caused by Mycobacterium tuberculosis; affects the lungs; prevented by the BCG vaccine.
- Cholera — caused by Vibrio cholerae; spread by contaminated water; causes severe watery diarrhoea.
- Typhoid — caused by Salmonella typhi; spread by contaminated food and water; affects the intestine.
- Tetanus — bacteria enter through wounds; cause painful muscle stiffness (“lockjaw”).
- Plague — spread by the bite of the rat flea.
- Diphtheria, whooping cough (pertussis), leprosy — other important bacterial diseases.
Cholera and typhoid are water-borne diseases — clean drinking water and good sanitation are the best prevention.
Protozoan and Vector-Borne Diseases
A vector is a living carrier (usually an insect) that transfers a pathogen from one host to another without itself causing the disease. Mosquitoes are the most important vectors in NDA questions.
Malaria — the classic example
Malaria is caused by the protozoan Plasmodium and spread by the bite of the female Anopheles mosquito. It affects red blood cells and the liver, causing repeated fever and chills.
Other vector-borne diseases
- Dengue, chikungunya, yellow fever, Zika — spread by the Aedes mosquito.
- Filariasis (elephantiasis) — caused by a worm, spread by the Culex mosquito.
- Kala-azar (leishmaniasis) — spread by the sandfly.
- Plague — spread by the rat flea.
- Sleeping sickness — spread by the tsetse fly.
Memorise the mosquito split: Anopheles → malaria, Aedes → dengue/chikungunya/yellow fever, Culex → filariasis. This single line answers many PYQs.
Deficiency Diseases
Some diseases are caused not by germs but by the lack of a particular vitamin or mineral in the diet. These are non-communicable and are cured simply by adding the missing nutrient back to the diet. A balanced diet containing fruits, vegetables, milk and pulses prevents almost all of them.
Vitamin deficiencies
- Vitamin A — deficiency causes night blindness; found in carrots, green vegetables.
- Vitamin B1 (thiamine) — deficiency causes beri-beri.
- Vitamin C (ascorbic acid) — deficiency causes scurvy (bleeding gums); found in citrus fruits, amla.
- Vitamin D — deficiency causes rickets in children; made in skin by sunlight.
- Vitamin K — needed for blood clotting; deficiency causes excessive bleeding.
Mineral deficiencies
- Iron — deficiency causes anaemia.
- Iodine — deficiency causes goitre (swelling of the thyroid).
- Calcium — needed for strong bones and teeth.
Link each vitamin to its deficiency with a quick chant: A-night blindness, C-scurvy, D-rickets, B1-beri-beri. These four are the most repeated.
Hereditary and Lifestyle Diseases
A few important non-communicable diseases come from your genes or from your habits.
Hereditary (genetic) diseases
These pass from parents to children through genes. Key examples: haemophilia (blood does not clot), colour blindness (cannot tell red from green), sickle-cell anaemia, and thalassaemia. Haemophilia and colour blindness are sex-linked and appear more often in males.
Lifestyle diseases
These build up over years due to diet, stress and lack of exercise: diabetes mellitus (high blood sugar from low insulin), hypertension (high blood pressure), obesity and many forms of heart disease. Cancer is the uncontrolled division of body cells.
Diabetes is linked to the hormone insulin, made by the pancreas. Too little insulin → sugar piles up in the blood.
Immunity, Vaccines and Prevention
The body defends itself using the immune system. White blood cells (WBCs) attack invaders, and special proteins called antibodies neutralise specific pathogens.
What is a vaccine?
A vaccine contains weakened or dead pathogens (or their parts). When given, it trains the immune system to make antibodies in advance, so that the real germ is destroyed quickly if it ever attacks. This is active immunity because the body itself makes the antibodies, and the protection often lasts for years. When ready-made antibodies are injected directly (as in anti-snake-venom or anti-tetanus injections), the protection is immediate but short-lived — this is called passive immunity. The idea of vaccination was pioneered by Edward Jenner, who developed the smallpox vaccine, and smallpox is so far the only human disease that has been completely eradicated worldwide.
- BCG — against tuberculosis.
- OPV — oral polio vaccine.
- DPT — against diphtheria, pertussis (whooping cough) and tetanus.
Simple prevention
- Drink safe, clean water and eat fresh food.
- Maintain hygiene and sanitation; wash hands.
- Control mosquitoes by removing stagnant water.
- Get the recommended vaccinations on time.
A vaccine prevents a disease; it does not cure one already in progress. Treatment of an existing infection uses medicines (antibiotics or antivirals), not vaccines.
Worked Example
Let us classify a set of diseases the way an examiner expects, sorting by cause and mode of spread.
Classify: malaria, scurvy, tuberculosis, haemophilia by their cause.
So two are communicable (malaria, TB) and two are non-communicable (scurvy, haemophilia). Notice how each answer comes from identifying the cause first — pathogen, missing nutrient, or gene. Whenever the examiner gives you a list to sort, start by asking “is there a germ involved?”; if yes, it is communicable, and if no, look for a deficiency, a gene or a lifestyle reason.
Previous-Year Style Question
NDA questions on this topic are usually one-line matching or single-fact questions. Here is a representative one.
Q. Which one of the following diseases is caused by a protozoan and transmitted by the female Anopheles mosquito?
Answer: Malaria. It is caused by the protozoan Plasmodium, carried by the female Anopheles mosquito. Dengue and chikungunya are viral (Aedes mosquito), while filariasis is caused by a worm (Culex mosquito) — so malaria is the only correct match.
Quick Revision
- Pathogen = disease-causing germ: virus, bacterium, protozoan, fungus or worm.
- Communicable diseases spread; non-communicable ones do not.
- Malaria → Plasmodium → female Anopheles; Dengue → Aedes; Filariasis → Culex.
- TB, cholera, typhoid = bacterial; polio, hepatitis, AIDS, rabies = viral.
- Deficiency: A → night blindness, C → scurvy, D → rickets, B1 → beri-beri, iodine → goitre.
- Hereditary: haemophilia, colour blindness; lifestyle: diabetes (insulin), hypertension.
- Vaccines give advance immunity; antibiotics work on bacteria, not viruses.
Master the disease-pathogen-vector table and the vitamin-deficiency list, and you will comfortably handle nearly every NDA question from this chapter.
Frequently asked questions
What is the difference between a pathogen and a vector?
A pathogen is the actual disease-causing organism, such as the Plasmodium that causes malaria. A vector is the living carrier, like the female Anopheles mosquito, that transfers the pathogen from one host to another but does not itself cause the disease.
Why don't antibiotics work against viral diseases?
Antibiotics target structures and processes found in bacteria, such as their cell walls. Viruses have no such structures and multiply inside the host's own cells, so antibiotics have no effect. Viral diseases are managed with antiviral drugs or prevented by vaccines.
Which mosquito causes which disease?
The female Anopheles mosquito spreads malaria, the Aedes mosquito spreads dengue, chikungunya and yellow fever, and the Culex mosquito spreads filariasis. This split is one of the most frequently asked points in NDA Biology.
Are deficiency diseases contagious?
No. Deficiency diseases like scurvy, rickets and goitre are caused by the lack of a vitamin or mineral in the diet, not by germs. They are non-communicable and are cured by supplying the missing nutrient.
How does a vaccine protect us?
A vaccine contains weakened or dead pathogens that train the immune system to produce antibodies in advance. If the real pathogen later attacks, the body recognises and destroys it quickly, giving protection known as active immunity.
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