Immunity & Human Diseases — Bacterial, Viral & Other

Why This Chapter Matters

Open any SSC CGL, CHSL, MTS, or RRB NTPC paper from the last five years and you will find at least one question asking which organism causes typhoid, or which vitamin prevents scurvy's cousin diseases, or what type of immunity a newborn gets from its mother's milk. This single topic, diseases and immunity, has quietly become one of the most repeated General Awareness themes across SSC and RRB exams, often worth 2-3 marks in a single paper. Banking exams touch it less, but SSC and RRB Group D papers lean on it hard.

Here is the shape of what is coming. You will learn how your body defends itself (innate versus acquired immunity), how vaccines exploit that defence system, and then the centerpiece of this chapter: a complete table pairing every major disease with its pathogen, how it spreads, and how to stop it. That table alone is worth memorising cold, because exam-setters love picking one row and asking a direct question about it.

The single biggest mistake aspirants make here is mixing up active and passive immunity, and separately, mixing up bacterial and viral diseases when a question lists four options and expects you to spot the odd one out. Students see "fever" and "spreads by mosquito" and guess without checking whether the culprit is a bacterium, a virus, or a protozoan. This chapter fixes that confusion permanently by grouping diseases strictly by pathogen type, the same way exam-setters group their options.

1. What Is Immunity?

Immunity is your body's ability to resist disease-causing organisms, called pathogens. Think of your body as a housing society with a security system. Some security is built into the gate itself, guards who stop every stranger regardless of who they are. Some security is built after a specific troublemaker is identified and a photo is circulated to every guard on duty. Your immune system works exactly this way, in two broad layers.

Innate (Non-Specific) Immunity

This is the immunity you are born with. It does not target one particular germ; it resists all germs in a general way. It has four lines of defence:

1. Physical barriers — skin, mucous membranes, the cornea of the eye.
2. Physiological barriers — stomach acid (hydrochloric acid kills most swallowed bacteria), tears and saliva (contain the enzyme lysozyme), the acidic pH of the skin.
3. Cellular barriers — white blood cells such as neutrophils, monocytes, natural killer (NK) cells, and macrophages that engulf and destroy pathogens.
4. Cytokine barriers — cells infected by a virus secrete proteins called interferons, which protect nearby uninfected cells from the same viral attack.

Exam trap: Innate immunity is present from birth and stays roughly constant. It does NOT "remember" a pathogen. Students often confuse this with acquired immunity, which does remember.

Acquired (Specific) Immunity

This immunity develops during your lifetime, targets a specific pathogen, and improves on repeat exposure. It comes in two types, and this pair is one of the most-asked concepts in the whole chapter.

Active immunity: your own body produces antibodies after exposure to a pathogen or a vaccine. It takes time to build up but lasts long, sometimes a lifetime. Example: catching chickenpox once and never catching it again, or getting vaccinated.

Passive immunity: ready-made antibodies are given to you or transferred to you from outside. It acts immediately but does not last long, because your own body never learned to make those antibodies. Example: a newborn baby receiving antibodies through the mother's placenta before birth and through colostrum (the first breast milk) after birth. An anti-snake-venom injection is another classic example of passive immunity.

Memory hook: Active immunity means your body is "actively" working, making its own antibodies, so it is slow to start but long-lasting, like training your own team. Passive immunity means you are a "passive" receiver of someone else's ready-made antibodies, fast but temporary, like borrowing a neighbour's tools for one job.

Exam trap: A vaccine gives ACTIVE immunity (your body makes antibodies in response to a weakened or killed pathogen), while an antiserum injection (like anti-rabies serum given after a dog bite in some protocols, or anti-snake-venom) gives PASSIVE immunity (ready-made antibodies injected directly). Do not assume every injection is passive immunity.

2. Antigens and Antibodies

An antigen is any foreign substance, usually a protein on a pathogen's surface, that triggers an immune response. An antibody (also called immunoglobulin) is a Y-shaped protein produced by B-lymphocytes that binds specifically to one antigen, like a lock matching one key. This lock-and-key specificity is why a measles vaccine protects only against measles and not against mumps.

Lymphocytes, a type of white blood cell, run this specific defence. B-cells mature in the bone marrow and produce antibodies. T-cells mature in the thymus gland (hence "T") and directly attack infected cells or help coordinate the immune response. This is also why the thymus, though it shrinks with age, is called a primary lymphoid organ alongside bone marrow.

3. Vaccination — History and How It Works

A vaccine contains a weakened, killed, or partial version of a pathogen. It cannot cause full-blown disease, but it is enough to make your immune system produce antibodies and, crucially, memory cells. If the real pathogen ever attacks later, your body already has the blueprint and responds fast, before you even fall sick.

Edward Jenner, an English physician, is credited with developing the first vaccine in 1796, using cowpox material to protect against smallpox. This is where the word "vaccine" comes from, the Latin vacca meaning cow. Smallpox was declared eradicated worldwide by the World Health Organization in 1980, the only human disease eradicated through vaccination so far. Louis Pasteur, decades later, developed vaccines for rabies and anthrax, and his work laid the modern foundation of immunology (he also gave us the germ theory of disease and the process of pasteurisation).

Exam trap: Jenner is linked to smallpox, Pasteur is linked to rabies and anthrax. Papers frequently swap these two names in wrong options. Do not confuse "father of vaccination" (Jenner) with "father of microbiology/germ theory" (Pasteur).

India's Universal Immunization Programme (UIP), one of the largest public health programmes in the world, protects children against diseases including tuberculosis (BCG vaccine), polio, diphtheria, pertussis, tetanus, hepatitis B, measles, and rubella. If you grew up in India, the small round scar many people carry on their upper arm is from the BCG vaccine, given at birth.

4. Human Diseases — Classified by Cause

Diseases that spread from one person to another are communicable (infectious) diseases, caused by pathogens. Diseases that do not spread from person to person, caused by internal body dysfunction, nutrient deficiency, or lifestyle, are non-communicable diseases (diabetes, cancer, heart disease). This chapter's main job is to sort communicable diseases correctly by the type of pathogen behind them, because that classification is exactly how SSC and RRB frame their questions.

Four pathogen types cause almost every communicable disease you will be tested on:

• Bacteria — single-celled organisms without a nucleus. Most bacterial diseases can be treated with antibiotics.
• Viruses — non-cellular particles that can only multiply inside a living host cell. Antibiotics do NOT work on viruses; that is a favourite exam trap.
• Protozoa — single-celled organisms with a nucleus, often spread by insect vectors.
• Fungi — spore-producing organisms, usually causing skin and surface infections in humans.

Exam trap: "Antibiotics cure viral fever" is a false statement that appears often in assertion-reason or true/false style questions. Antibiotics target bacterial cell walls and processes; viruses have no such cell wall, so antibiotics are useless against them. This is precisely why doctors do not prescribe antibiotics for common viral flu.

The Master Table: Disease, Pathogen, Transmission, Prevention

This is the single most exam-relevant table in this book. Study it in blocks by pathogen type, since that is how options are usually grouped in a question.

Bacterial Diseases

Disease	Causative Organism	Mode of Transmission	Prevention/Control
Tuberculosis (TB)	Mycobacterium tuberculosis	Airborne droplets (coughing, sneezing)	BCG vaccine, DOTS therapy, covering mouth while coughing
Typhoid	Salmonella typhi	Contaminated food and water	Typhoid vaccine, safe drinking water, hygiene
Cholera	Vibrio cholerae	Contaminated water, poor sanitation	Oral rehydration, clean water supply, sanitation
Diphtheria	Corynebacterium diphtheriae	Airborne droplets	DPT vaccine
Whooping cough (Pertussis)	Bordetella pertussis	Airborne droplets	DPT vaccine
Tetanus	Clostridium tetani	Deep wounds contaminated with soil/dust	TT vaccine, wound cleaning
Leprosy	Mycobacterium leprae	Prolonged close contact	Multi-drug therapy (MDT), early detection
Plague	Yersinia pestis	Rat flea bite	Rodent and flea control, sanitation
Pneumonia	Streptococcus pneumoniae	Airborne droplets	Pneumococcal vaccine, hygiene

Viral Diseases

Disease	Causative Organism	Mode of Transmission	Prevention/Control
Common cold	Rhinovirus	Airborne droplets, contact	Hygiene, rest (no cure, only symptom relief)
Influenza (Flu)	Influenza virus	Airborne droplets	Annual flu vaccine, hygiene
Measles	Measles virus	Airborne droplets	MMR vaccine
Mumps	Mumps virus	Airborne droplets, saliva	MMR vaccine
Rubella (German measles)	Rubella virus	Airborne droplets	MMR vaccine
Chickenpox	Varicella-zoster virus	Airborne droplets, direct contact	Varicella vaccine
Poliomyelitis	Poliovirus	Contaminated food/water (faecal-oral route)	Oral polio vaccine (OPV)
Hepatitis A	Hepatitis A virus	Contaminated food/water	Hepatitis A vaccine, hygiene
Hepatitis B	Hepatitis B virus	Blood, body fluids, mother to child	Hepatitis B vaccine
Rabies	Rabies virus	Bite of infected animal (dog, monkey, bat)	Anti-rabies vaccine, wound washing
Dengue fever	Dengue virus	Aedes aegypti mosquito bite	Vector control, avoiding stagnant water
Chikungunya	Chikungunya virus	Aedes aegypti mosquito bite	Vector control
AIDS	HIV (Human Immunodeficiency Virus)	Blood, sexual contact, mother to child	Safe practices, screened blood, no vaccine yet
COVID-19	SARS-CoV-2	Airborne droplets, contact	Vaccination, masks, hygiene

Protozoan Diseases

Disease	Causative Organism	Mode of Transmission	Prevention/Control
Malaria	Plasmodium species (P. vivax, P. falciparum, others)	Female Anopheles mosquito bite	Vector control, mosquito nets, antimalarial drugs
Amoebiasis (Amoebic dysentery)	Entamoeba histolytica	Contaminated food/water	Hygiene, safe drinking water
Kala-azar (Visceral leishmaniasis)	Leishmania donovani	Sandfly bite	Vector control
Sleeping sickness	Trypanosoma species	Tsetse fly bite	Vector control (mainly reported in Africa)
Giardiasis	Giardia lamblia	Contaminated water	Safe drinking water, hygiene

Fungal Diseases

Disease	Causative Organism	Mode of Transmission	Prevention/Control
Ringworm	Trichophyton / Microsporum species	Direct contact, shared clothing/towels	Personal hygiene, avoiding shared items
Athlete's foot	Trichophyton species	Direct/indirect contact, damp environments	Keeping feet dry, hygiene
Candidiasis	Candida albicans	Contact, overgrowth in weakened immunity	Hygiene, managing underlying conditions

Read this table the way you would read a railway timetable: not once, but enough times that your eye jumps straight to the right row when you see a keyword. If a question says "Aedes aegypti," your mind should jump straight to dengue or chikungunya, never malaria. If it says "female Anopheles," it can only mean malaria.

Memory hook (mosquito-borne diseases): Picture a mosquito clinic with three separate counters. "Anopheles handles Malaria" (both start with vowels A and M, an easy pairing), while "Aedes handles Dengue and Chikungunya" (Aedes bites in daylight, unlike the night-biting Anopheles). This day/night habit difference is itself a fact SSC has tested: Anopheles mosquitoes typically bite at night, Aedes mosquitoes typically bite during the day.

Exam trap: Do not confuse the pathogen with the vector. The mosquito is the vector (carrier), not the disease-causing organism. In malaria, the actual pathogen is the protozoan Plasmodium; the Anopheles mosquito only transports it from one person to another. A question asking "causative organism of malaria" wants Plasmodium, not "mosquito."

5. Transmission Routes, Grouped for Recall

Grouping by transmission route, rather than by pathogen, is a second useful lens exams sometimes use.

• Airborne (droplet) diseases: TB, diphtheria, whooping cough, common cold, flu, measles, mumps, rubella, chickenpox, COVID-19.
• Waterborne/foodborne (faecal-oral) diseases: typhoid, cholera, polio, hepatitis A, amoebiasis, giardiasis.
• Vector-borne diseases: malaria and kala-azar (via insects biting and injecting the pathogen), dengue and chikungunya (via Aedes mosquito), plague (via rat flea).
• Contact/blood-borne diseases: HIV/AIDS, hepatitis B, rabies (animal bite), ringworm.

Analogy: Think of these four routes as four different delivery services to your body. Airborne diseases arrive by "courier through the air" every time someone coughs near you. Waterborne diseases arrive through a "contaminated pipeline," your food and water supply. Vector-borne diseases have a "delivery agent," the mosquito or flea, that physically carries the pathogen door to door. Contact/blood-borne diseases require "direct hand-off," a bite, a needle, or bodily fluid exchange. Once you sort a disease into its delivery service, the correct prevention method almost writes itself: fix the pipeline for waterborne diseases, kill the delivery agent for vector-borne diseases, and so on.

6. Prevention Principles That Repeat Across Diseases

Notice a pattern in the tables above: prevention almost always mirrors transmission. Sanitation and safe drinking water block the faecal-oral route. Vector control (removing stagnant water, using nets, spraying) blocks mosquito and flea-borne diseases. Vaccination blocks specific viral and bacterial diseases regardless of route, because it works on your immune system rather than on the environment. Covering your mouth while coughing and general hygiene block airborne and contact diseases.

Vaccines available for bacterial diseases in this chapter: TB (BCG), diphtheria/pertussis/tetanus (DPT), typhoid, pneumonia (pneumococcal).

Vaccines available for viral diseases in this chapter: polio (OPV), measles/mumps/rubella (MMR), hepatitis A, hepatitis B, chickenpox (varicella), rabies, COVID-19, influenza.

No vaccine currently exists for malaria as a fully deployed routine option in India at the scale of the others, for HIV/AIDS, or for the common cold. This is a genuinely useful fact: exam-setters like to ask "for which of the following is NO vaccine available," and dengue and malaria are the two classic correct answers, since a malaria vaccine remains limited in rollout and dengue has no widely deployed vaccine in India either. HIV/AIDS also has no vaccine, only prevention and antiretroviral management.

7. Autoimmune Disorders and Allergies (Brief but Testable)

Occasionally the immune system attacks the body's own healthy cells by mistake, mistaking "self" for "non-self." This is an autoimmune disorder. Rheumatoid arthritis (attacking joints) and Type 1 diabetes (attacking insulin-producing pancreatic cells) are commonly cited examples.

An allergy is an exaggerated immune response to a normally harmless substance (an allergen), such as pollen, dust, or certain foods. The chemical histamine, released by the body during an allergic reaction, causes symptoms like sneezing, itching, and swelling. That is why anti-allergy medicines are called antihistamines.

Quick Revision — One-Line Facts

• Immunity is of two broad types: innate (non-specific, present at birth) and acquired (specific, develops over life).
• Acquired immunity has two forms: active (body makes its own antibodies, slow but long-lasting) and passive (ready-made antibodies received from outside, fast but short-lived).
• A newborn gets passive immunity from the mother via the placenta and through colostrum.
• B-lymphocytes produce antibodies; T-lymphocytes, matured in the thymus, coordinate and attack directly.
• Edward Jenner developed the first vaccine, for smallpox, in 1796, using cowpox.
• Louis Pasteur developed vaccines for rabies and anthrax and gave us germ theory.
• Smallpox is the only human disease officially declared eradicated worldwide, by WHO in 1980.
• Tuberculosis is caused by Mycobacterium tuberculosis and prevented by the BCG vaccine.
• Typhoid is caused by Salmonella typhi, spread through contaminated food and water.
• Cholera is caused by Vibrio cholerae, also a waterborne disease.
• Leprosy is caused by Mycobacterium leprae, treated with multi-drug therapy (MDT).
• Plague is caused by Yersinia pestis, transmitted by rat fleas.
• Malaria is caused by the protozoan Plasmodium, transmitted by the female Anopheles mosquito.
• Dengue and chikungunya are viral diseases transmitted by the Aedes aegypti mosquito.
• Anopheles mosquitoes generally bite at night; Aedes mosquitoes generally bite during the day.
• Kala-azar (visceral leishmaniasis) is caused by Leishmania donovani, transmitted by sandflies.
• AIDS is caused by HIV, spread through blood, sexual contact, and mother-to-child transmission; no vaccine exists yet.
• Rabies is a viral disease transmitted by the bite of an infected animal, mainly dogs.
• Ringworm and athlete's foot are fungal diseases spread by direct or indirect contact.
• Polio is caused by the poliovirus, spread through the faecal-oral route, prevented by OPV.
• Measles, mumps, and rubella are all prevented by a single combined vaccine: MMR.
• Antibiotics work only against bacterial infections, never against viral infections.
• Interferons are proteins secreted by virus-infected cells to protect neighbouring cells.
• Lysozyme, present in tears and saliva, is an enzyme that destroys bacterial cell walls.
• India's child vaccination programme is called the Universal Immunization Programme (UIP).
• Histamine is the chemical responsible for allergic reaction symptoms; antihistamines block it.
• Autoimmune disorders occur when the immune system attacks the body's own cells.
• Rheumatoid arthritis and Type 1 diabetes are commonly cited autoimmune disorders.
• COVID-19 is caused by the virus SARS-CoV-2, spread mainly through airborne droplets.
• Antigens trigger immune responses; antibodies are the specific proteins that neutralise them.
• Vector-borne diseases need a carrier organism, mosquito, flea, or fly, to transport the actual pathogen.

Memory Tables

Table 1: Immunity Types at a Glance

Type	Speed of Action	Duration	Source	Example
Innate	Immediate	Lifelong, constant	Present from birth	Skin, stomach acid, lysozyme
Acquired — Active	Slow (days to build)	Long-lasting, often lifelong	Body's own antibody production	Vaccination, recovering from an infection
Acquired — Passive	Immediate	Short-lived (weeks to months)	Ready-made antibodies from outside	Mother's colostrum, anti-snake-venom serum

Table 2: Vaccine Pioneers and Landmark Discoveries

Scientist	Contribution	Year/Note
Edward Jenner	First vaccine, for smallpox, using cowpox material	1796
Louis Pasteur	Vaccines for rabies and anthrax; germ theory of disease	Mid-to-late 1800s
World Health Organization	Declared smallpox globally eradicated	1980

Table 3: Pathogen Type vs Treatment Approach

Pathogen Type	Example Diseases	Typically Treated With
Bacteria	TB, typhoid, cholera, tetanus, leprosy	Antibiotics
Virus	Flu, measles, polio, HIV/AIDS, COVID-19	Antivirals/vaccination/supportive care (NOT antibiotics)
Protozoa	Malaria, amoebiasis, kala-azar	Antiprotozoal/antimalarial drugs
Fungi	Ringworm, athlete's foot, candidiasis	Antifungal drugs

Practice MCQs

Q1. Who is credited with developing the first vaccine, against smallpox, in 1796? (a) Louis Pasteur (b) Robert Koch (c) Edward Jenner (d) Alexander Fleming

Q2. Which of the following provides an example of passive immunity? (a) Recovering from chickenpox once and never catching it again (b) Antibodies received by a newborn through the mother's colostrum (c) Getting the BCG vaccine at birth (d) The body producing antibodies after a flu infection

Q3. Malaria is caused by which type of organism? (a) Bacterium (b) Virus (c) Protozoan (d) Fungus

Q4. Which mosquito is responsible for transmitting dengue and chikungunya? (a) Female Anopheles (b) Aedes aegypti (c) Culex (d) Tsetse fly

Q5. Antibiotics are effective against which of the following? (a) All viral infections (b) All fungal infections (c) Bacterial infections (d) All infections without exception

Q6. Which organ in the human body is where T-lymphocytes mature? (a) Bone marrow (b) Liver (c) Thymus (d) Spleen

Q7. Vibrio cholerae is the causative organism of which disease? (a) Typhoid (b) Cholera (c) Tuberculosis (d) Plague

Q8. Which disease is transmitted by the bite of an infected rat flea? (a) Kala-azar (b) Plague (c) Malaria (d) Dengue

Q9. Smallpox was declared eradicated worldwide by the WHO in which year? (a) 1972 (b) 1980 (c) 1990 (d) 2000

Q10. Which of the following diseases currently has NO widely available vaccine? (a) Measles (b) Hepatitis B (c) HIV/AIDS (d) Polio

Q11. Which enzyme, present in tears and saliva, forms part of the body's innate immunity by destroying bacterial cell walls? (a) Pepsin (b) Lysozyme (c) Amylase (d) Trypsin

Q12. Leprosy is caused by which organism and treated with which therapy? (a) Mycobacterium leprae; multi-drug therapy (MDT) (b) Mycobacterium tuberculosis; DOTS therapy (c) Plasmodium falciparum; antimalarial drugs (d) Trichophyton; antifungal cream

Q13. A student notes that a patient has fever, is bitten by a mosquito that bites mainly during daytime, and tests positive for a viral infection. Which disease does this most likely describe? (a) Malaria (b) Kala-azar (c) Dengue (d) Sleeping sickness

Q14. Which pair correctly matches the disease with its causative organism? (a) Diphtheria — Clostridium tetani (b) Amoebiasis — Entamoeba histolytica (c) Ringworm — Vibrio cholerae (d) Rabies — Salmonella typhi

Q15. An autoimmune disorder occurs when: (a) The immune system fails to respond to any pathogen (b) The immune system attacks the body's own healthy cells (c) A vaccine causes the original disease it was meant to prevent (d) Passive immunity lasts longer than active immunity

Answer Key

Q	Answer	One-line reason
Q1	(c) Edward Jenner	Jenner used cowpox material to create the first smallpox vaccine in 1796; Pasteur came later with rabies and anthrax vaccines.
Q2	(b) Colostrum antibodies	Ready-made antibodies transferred from another source is the definition of passive immunity; the other options all involve the body making its own antibodies (active immunity).
Q3	(c) Protozoan	Malaria is caused by the protozoan Plasmodium, transmitted by the mosquito, which is only the vector, not the pathogen.
Q4	(b) Aedes aegypti	Aedes aegypti transmits both dengue and chikungunya and typically bites during the day, unlike the night-biting Anopheles.
Q5	(c) Bacterial infections	Antibiotics target bacterial structures like the cell wall; viruses have no cell wall, so antibiotics cannot act on them.
Q6	(c) Thymus	T-lymphocytes are named for the thymus, where they mature, while B-lymphocytes mature in the bone marrow.
Q7	(b) Cholera	Vibrio cholerae is a comma-shaped bacterium spread through contaminated water, causing severe cholera.
Q8	(b) Plague	Plague is spread by the bite of a flea carried on infected rats, caused by the bacterium Yersinia pestis.
Q9	(b) 1980	The WHO officially certified global smallpox eradication in 1980, the only human disease eradicated this way so far.
Q10	(c) HIV/AIDS	No vaccine currently exists for HIV/AIDS; measles, hepatitis B, and polio all have well-established, widely used vaccines.
Q11	(b) Lysozyme	Lysozyme is a physiological barrier enzyme found in tears and saliva that breaks down bacterial cell walls.
Q12	(a) Mycobacterium leprae; MDT	Leprosy is caused by Mycobacterium leprae and treated with multi-drug therapy (MDT), distinct from TB's DOTS regimen.
Q13	(c) Dengue	A viral fever plus a daytime-biting mosquito points to Aedes-borne dengue, not the protozoan malaria or night-biting Anopheles diseases.
Q14	(b) Amoebiasis — Entamoeba histolytica	This is the only correctly matched pair; the other three swap organisms across unrelated diseases, a classic exam trick.
Q15	(b) Attacks own healthy cells	Autoimmune disorders like rheumatoid arthritis happen when the immune system misidentifies the body's own tissue as a threat.