This MCQ module is based on: Common Diseases
TOPIC 30 OF 64
Common Diseases
🎓 Class 12
Biology
CBSE
Theory
Ch 7 – Human Health and Disease
⏱ ~14 min
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Common Diseases
8.1 What is Health?
Health is not merely the absence of disease or physical fitness. The World Health Organization defines it as a state of complete physical, mental and social well-being. Healthy people are more productive, live longer and contribute to lower infant and maternal mortality. Health is influenced by three broad categories of factors:
- Genetic disorders — deficiencies a child is born with or inherits from parents.
- Infections — diseases caused by pathogens.
- Lifestyle — food, water, rest, exercise, habits.
When the functioning of one or more organs/systems is adversely affected — characterised by various signs and symptoms — we say a person has a disease. Diseases are broadly grouped as:
- Infectious diseases — easily transmitted between individuals (e.g., typhoid, AIDS).
- Non-infectious diseases — not directly transmitted (e.g., cancer, diabetes).
8.2 Common Diseases in Humans
A wide range of organisms — bacteria, viruses, fungi, protozoans, helminths — cause diseases. Such organisms are called pathogens. Most parasites are pathogens because they harm the host. Pathogens have to adapt to the internal environment of the host — for example, gut pathogens must survive the low pH of the stomach and the digestive enzymes.
8.3 Bacterial Diseases
Typhoid (Salmonella typhi)
Pathogen: Salmonella typhi — a Gram-negative bacterium.
Mode of transmission: Contaminated food and water; enters the small intestine and migrates to other organs through blood.
Symptoms: Sustained high fever (39°–40°C), weakness, stomach pain, constipation, headache, loss of appetite. Intestinal perforation and death may occur in severe cases.
Diagnosis: Widal test — detects antibodies in patient's blood.
Typhoid Mary (historical): Mary Mallon was a cook in early 20th-century New York who was a healthy carrier of Salmonella typhi. She unknowingly transmitted typhoid through the food she prepared to dozens of people across several households over many years. The case established the concept of asymptomatic carriers in epidemiology.
Pneumonia
Pathogens: Streptococcus pneumoniae and Haemophilus influenzae.
Site of infection: Alveoli (air sacs) of the lungs.
Symptoms: Fever, chills, cough, headache; alveoli fill with fluid leading to severe breathing problems; in severe cases lips and fingernails turn gray to bluish.
Mode of transmission: Inhalation of droplets/aerosols from an infected person; sharing glasses, utensils.
Other bacterial diseases: dysentery, plague, diphtheria.
8.4 Viral Diseases
Common Cold (Rhinovirus)
Pathogen: Rhino viruses.
Site of infection: Nose and respiratory passages (NOT the lungs).
Symptoms: Nasal congestion and discharge, sore throat, hoarseness, cough, headache, tiredness — typically last 3–7 days.
Mode of transmission: Droplets from cough/sneeze (inhaled directly), or contaminated objects (pens, books, cups, doorknobs, computer keyboard).
8.5 Protozoan Diseases
Malaria (Plasmodium)
Pathogen: Plasmodium species. Three NCERT species: P. vivax, P. malariae, P. falciparum. Malignant malaria caused by P. falciparum is the most serious and can be fatal.
Vector: Female Anopheles mosquito.
Hosts: Two — human (intermediate) and mosquito (definitive).
Plasmodium life cycle steps:
- Plasmodium enters human body as sporozoites through the bite of an infected female Anopheles mosquito.
- Parasites multiply within liver cells.
- Parasites attack red blood cells (RBCs) and multiply.
- RBCs rupture, releasing the toxic substance haemozoin — responsible for recurring chill and high fever every 3–4 days.
- When another female Anopheles bites the infected person, gametocytes enter the mosquito with the blood meal.
- In the mosquito, sexual reproduction occurs; sporozoites form and migrate to the salivary glands.
- When this mosquito bites another human, sporozoites are introduced — completing the cycle.
Amoebiasis (Entamoeba histolytica)
Pathogen: Entamoeba histolytica.
Site: Large intestine.
Disease name: Amoebic dysentery.
Symptoms: Constipation, abdominal pain and cramps, stools with excess mucus and blood clots.
Vector: Houseflies — mechanical carriers transferring parasites from faeces to food.
Mode of transmission: Contaminated food and drinking water.
8.6 Helminthic Diseases
Ascariasis (Ascaris lumbricoides)
Pathogen: Ascaris — the common round worm. Intestinal parasite.
Symptoms: Internal bleeding, muscular pain, fever, anaemia, blockage of intestinal passage.
Mode of transmission: Eggs in faeces of infected persons contaminate soil, water, plants. Healthy person acquires infection through contaminated water, vegetables, fruits.
Filariasis / Elephantiasis (Wuchereria)
Pathogen: Wuchereria bancrofti and W. malayi — filarial worms.
Site: Lymphatic vessels of lower limbs (often genital organs too).
Symptoms: Chronic inflammation; gross deformities and swelling of affected organs (hence the name elephantiasis).
Vector: Female mosquitoes.
8.7 Fungal Diseases
Ringworm
Pathogens: Fungi of the genera Microsporum, Trichophyton and Epidermophyton.
Site: Skin, nails, scalp.
Symptoms: Dry, scaly lesions; intense itching. Heat and moisture promote growth — fungi thrive in skin folds (groin, between toes).
Mode of transmission: Soil, towels, clothes, combs of infected persons.
8.8 Disease–Pathogen Summary Table
| Disease | Pathogen Type | Pathogen (species) | Mode of Transmission |
|---|---|---|---|
| Typhoid | Bacterium | Salmonella typhi | Contaminated food/water |
| Pneumonia | Bacterium | Streptococcus pneumoniae, Haemophilus influenzae | Droplets, shared utensils |
| Common Cold | Virus | Rhinoviruses | Droplets, contact |
| Malaria | Protozoan | Plasmodium spp. | Female Anopheles mosquito bite |
| Amoebiasis | Protozoan | Entamoeba histolytica | Contaminated food/water (housefly carrier) |
| Ascariasis | Helminth (roundworm) | Ascaris lumbricoides | Contaminated food/water/soil |
| Filariasis / Elephantiasis | Helminth (filarial worm) | Wuchereria bancrofti, W. malayi | Female mosquito bite |
| Ringworm | Fungus | Microsporum, Trichophyton, Epidermophyton | Soil, towels, combs |
8.9 Prevention and Control of Infectious Diseases
Personal hygiene:
For vector-borne diseases (malaria, filariasis, dengue, chikungunya):
- Keep the body clean.
- Drink clean water; eat clean food, fruits and vegetables.
- Proper disposal of waste and excreta.
- Periodic cleaning and disinfection of water reservoirs, pools, cesspools and tanks.
- Standard hygiene practices in public catering.
For vector-borne diseases (malaria, filariasis, dengue, chikungunya):
- Eliminate vector breeding sites — avoid stagnant water in and around homes.
- Regularly clean household coolers.
- Use mosquito nets.
- Introduce larvivorous fish like Gambusia in ponds.
- Spray insecticides in ditches, drainage areas, swamps.
- Use wire mesh on doors and windows.
Modern biology has supplied vaccines, antibiotics and diagnostic tests that have controlled (and in the case of smallpox, eradicated) many infectious diseases. Polio, diphtheria, pneumonia and tetanus are now controlled by vaccination programmes.
Interactive: Disease–Pathogen Matcher
Match a disease to its pathogen and learn the transmission route:
Pathogen: —
Type: —
Transmission: —
Key symptom: —
Activity 8.1 — Predict-Observe-Explain: Stagnant Water Trial
Setup: A village reports many malaria cases each monsoon. A health team finds discarded coconut shells, plastic containers and tyre piles in the compound — all collecting rain water.
Predict: Why is the malaria incidence high in this village? What single most-effective measure would you recommend, and why?
Diagnosis: Stagnant water in containers is the perfect breeding site for Anopheles mosquito larvae. Adult mosquitoes emerging from these sites bite villagers and transmit Plasmodium.
Most effective measure: Eliminate breeding sites — empty out all stagnant water from coconut shells, tyres and containers; cover stored water; fill or treat permanent water bodies. This breaks the life cycle at the larval stage before mosquitoes emerge.
Why this works best: The mosquito MUST lay eggs in water; without water, no mosquitoes; without mosquitoes, no transmission. Treating sick people only after infection is reactive; preventing mosquito emergence is preventive. Additional measures: nets, mesh screens, larvivorous fish (Gambusia) in ornamental ponds.
Worked Examples
Worked Example 1: Differentiate the modes of transmission of malaria and amoebiasis.
Malaria: Transmitted by the bite of a female Anopheles mosquito. The mosquito is the biological vector; the parasite (Plasmodium) actually multiplies inside the mosquito. Without the mosquito vector, transmission cannot occur.
Amoebiasis: Transmitted via contaminated food or water (faecal–oral route). Houseflies act only as mechanical carriers — they carry Entamoeba cysts from faeces to food but do not biologically host the parasite.
Key difference: Mosquitoes are essential biological hosts for Plasmodium; houseflies are merely passive transporters for Entamoeba.
Amoebiasis: Transmitted via contaminated food or water (faecal–oral route). Houseflies act only as mechanical carriers — they carry Entamoeba cysts from faeces to food but do not biologically host the parasite.
Key difference: Mosquitoes are essential biological hosts for Plasmodium; houseflies are merely passive transporters for Entamoeba.
Worked Example 2: Why is the recurring fever in malaria associated with the rupture of RBCs?
Inside RBCs, Plasmodium multiplies asexually until the cell bursts. The rupture releases parasites and the toxic byproduct haemozoin (a dark pigment formed from haemoglobin breakdown).
Haemozoin triggers a strong immune response — release of cytokines (especially TNF) — which causes:
Haemozoin triggers a strong immune response — release of cytokines (especially TNF) — which causes:
- Chills (the patient feels cold).
- High fever (39°–40°C).
- Sweating phase as fever breaks.
Worked Example 3: Smallpox has been eradicated globally but typhoid persists. Explain the difference.
Why smallpox could be eradicated:
- Caused by a single virus (Variola) with no animal reservoir — humans were the only host.
- Highly visible symptoms — easy to identify and quarantine cases.
- Effective vaccine (cowpox-based) lasting for life.
- Coordinated WHO campaign 1967–1980.
- Bacterial (Salmonella typhi) — bacteria can survive in environment (water, food).
- Asymptomatic carriers (like Typhoid Mary) keep spreading the disease silently.
- Vaccines exist but offer only partial, time-limited immunity.
- Strongly linked to poor sanitation — eradication requires universal access to clean water and sewage, which many regions still lack.
Competency-Based Questions
Q1. The causative organism of malaria is: L1 Remember
Answer: (b) Plasmodium. A protozoan. Three NCERT species: P. vivax, P. malariae, P. falciparum. P. falciparum causes the most severe (malignant) malaria.
Q2. Match disease to pathogen type: L2 Understand
1 → Bacterium (Salmonella typhi)
2 → Virus (Rhinovirus)
3 → Fungus (Microsporum, Trichophyton, Epidermophyton)
4 → Helminth (Wuchereria)
2 → Virus (Rhinovirus)
3 → Fungus (Microsporum, Trichophyton, Epidermophyton)
4 → Helminth (Wuchereria)
Q3. Short Answer: Name the two hosts in the life cycle of Plasmodium. Which is the vector? L2 Understand
Two hosts: (1) Human (intermediate host — asexual phase in liver and RBCs); (2) Female Anopheles mosquito (definitive host — sexual reproduction).
Vector: The female Anopheles mosquito — she transmits the parasite from infected to healthy humans. Only the female bites because she needs blood for egg production; males feed on plant juices.
Vector: The female Anopheles mosquito — she transmits the parasite from infected to healthy humans. Only the female bites because she needs blood for egg production; males feed on plant juices.
Q4. Analyse: Houseflies do not transmit malaria but are blamed for amoebiasis. Why? L4 Analyse
Answer: The two pathogens have different transmission requirements.
- Plasmodium needs to biologically develop inside a specific mosquito (Anopheles) to form sporozoites. Houseflies lack the right physiology — Plasmodium cannot survive or develop in them.
- Entamoeba histolytica spreads via the faecal–oral route. Houseflies feed on faeces and food alike; they passively carry cysts on their feet/mouthparts. Cysts are resistant and remain infective.
Q5. HOT (Create): Design a public-awareness poster (3 key actions) to reduce malaria in a coastal Indian village during monsoon. L6 Create
Sample 3-point Awareness Poster:
- "Eliminate Stagnant Water" — Empty every container, coconut shell, tyre, plate. No water → no mosquito breeding.
- "Sleep Under a Net" — Use insecticide-treated bed nets. The Anopheles mosquito bites mainly at night. Protect children especially.
- "Fever > 3 days? Get tested!" — Visit the nearest PHC for a blood smear. Early diagnosis + chloroquine/artemisinin treatment prevents severe complications.
Assertion–Reason Questions
Choose: (A) Both true, R explains A. (B) Both true, R doesn't explain A. (C) A true, R false. (D) A false, R true.
A: Malaria patients show recurring chills and fever every 3–4 days.
R: The rupture of infected RBCs releases the toxic substance haemozoin in synchronous waves.
Answer: (A). Both true; R explains A. Haemozoin and inflammatory cytokines trigger the fever spike. The 48–72 hour cycle matches the parasite's RBC reproduction time.
A: Common cold typically lasts 3–7 days and is caused by rhinoviruses.
R: Rhinoviruses infect the lungs and lead to severe respiratory distress.
Answer: (C). A is TRUE. R is FALSE — rhinoviruses infect the nose and upper respiratory passages, NOT the lungs. Symptoms are mild (nasal congestion, cough) and self-limiting in healthy people.
A: Filariasis can cause gross deformities of the lower limbs (elephantiasis).
R: Filarial worms (Wuchereria) live in lymphatic vessels and cause chronic inflammation over many years.
Answer: (A). Both true; R explains A. Chronic lymphatic blockage causes fluid accumulation and skin thickening, producing the characteristic massive swelling.
Frequently Asked Questions - Common Diseases
What is the main concept covered in Common Diseases?
In NCERT Class 12 Biology Chapter on Human Health and Disease, "Common Diseases" covers the core biological structures, processes, and pathways students need for board exam success. The MyAiSchool lesson explains the topic with definitions, labelled diagrams, comparison tables, and interactive simulations. Scientific terminology and physiological/genetic significance are highlighted throughout to build conceptual depth aligned with CBSE 2025-26 syllabus.
How is Common Diseases useful in real-life or applied biology?
Real-life applications of "Common Diseases" from NCERT Class 12 Biology Human Health and Disease include medical diagnostics, agriculture, biotechnology, public health, evolutionary insights, and ecological monitoring. The MyAiSchool lesson links every biological concept to a tangible application so students see biology as a problem-solving framework for living systems and real-world challenges.
What are the key terms students should memorize for Common Diseases?
Key terms in "Common Diseases" (NCERT Class 12 Biology Human Health and Disease) are tabulated in the MyAiSchool key-terms grid. Students should memorize each term with its precise definition, function, and example. Terminology is high-yield in CBSE board exams — 1-mark MCQs and 2-mark short answers test definitions directly. The Summary section provides a printable quick-reference card.
How does this part connect to other parts of the chapter?
NCERT Class 12 Biology Human Health and Disease is structured so each part builds biological understanding sequentially. "Common Diseases" connects to neighbouring parts via shared mechanisms, structural hierarchies, and physiological processes. The MyAiSchool lesson cross-references related concepts with internal links so students can navigate the whole chapter as one connected biological story rather than disconnected fragments.
What types of CBSE board questions come from Common Diseases?
CBSE board questions from "Common Diseases" typically include: (1) 1-mark MCQs on definitions and processes, (2) 2-mark short-answer differences/comparisons, (3) 3-mark labelled-diagram questions, (4) 5-mark long-answer essays combining mechanism + diagram + significance. The MyAiSchool lesson tags each Competency-Based Question (CBQ) with Bloom level (L1-L6) so students know how to study for each weight.
How can students use the interactive simulation effectively?
The interactive simulation in the "Common Diseases" lesson allows students to explore biological processes, classifications, or pathways using selectors and sliders, with live visual feedback. To use it effectively: (1) explore each option/state, (2) compare with textbook diagrams, (3) note the function/outcome changes, (4) try the integrated practice quiz. The simulation reinforces visual-spatial understanding that pure text-based study cannot.
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