NCERT · Class XII Biology · Chapter 7
Human Health
& Disease
Complete chapter notes — every concept, table, flow, and key takeaway explained in full detail.
11
Topics
8
Diseases
5
Pathogen Types
12
Quiz Questions
01
Disease & Health
When one or more organs or body systems are adversely affected, showing various signs and symptoms, we are said to have a disease — meaning we are not in a state of good health.
Classification of Diseases
Diseases are broadly grouped into two categories:
1. Infectious Diseases
- Easily transmitted from one person to another
- Very common — nearly everyone suffers from them at some point
- Some are fatal, e.g., AIDS
2. Non-Infectious Diseases
- Not transmitted between individuals
- Cancer is the major cause of death in this category
Additional Health Hazards
Beyond diseases, drug and alcohol abuse also adversely affect human health, making them significant public health concerns.
Key Takeaway
| Feature | Infectious Diseases | Non-Infectious Diseases |
|---|---|---|
| Transmission | Person to person | Not transmitted |
| Example | AIDS, common cold | Cancer, diabetes |
| Commonality | Very common | Varies |
Key Takeaway
This forms the foundation of understanding human health and disease, which is a core topic in biology.
02
Pathogens & Disease-Causing Organisms
What is a Pathogen?
A wide range of organisms — bacteria, viruses, fungi, protozoans, helminths, etc. — could cause diseases in man. Such disease-causing organisms are called pathogens. Most parasites are therefore pathogens as they cause harm to the host by living in (or on) them.
Types of Pathogenic Organisms
| Group | Examples |
|---|---|
| Bacteria | Salmonella, Mycobacterium |
| Viruses | Influenza virus, HIV |
| Fungi | Candida, Aspergillus |
| Protozoans | Plasmodium, Entamoeba |
| Helminths | Tapeworms, Roundworms |
How Pathogens Cause Disease
Once they enter the body, pathogens follow a sequence:
Enter the body
↓
Multiply
↓
Interfere with vital activities
↓
Cause damage
This results in two types of damage:
- Morphological damage — structural/physical changes in tissues or organs
- Functional damage — disruption of normal body functions
Adaptation of Pathogens
Pathogens must adapt to the host's internal environment to survive. For example, pathogens entering the gut must:
- Survive in the stomach at low pH (highly acidic)
- Resist digestive enzymes that would otherwise destroy them
This shows that pathogens are highly evolved organisms with specialized survival mechanisms.
Key Takeaway
Pathogens are not just harmful organisms — they are biologically sophisticated entities that have evolved strategies to enter, survive, multiply, and cause damage within a host, making understanding them essential for disease prevention and control.
03
Typhoid Fever
Causative Organism
| Feature | Detail |
|---|---|
| Pathogen | Salmonella typhi |
| Type | Pathogenic Bacterium |
| Disease | Typhoid fever in human beings |
Mode of Transmission / Entry
- Pathogens enter through the small intestine
- Source: Contaminated food and water
- After entry, they migrate to other organs through blood (making it a systemic disease)
Symptoms of Typhoid Fever
| Symptom | Details |
|---|---|
| High Fever | Sustained 39°C to 40°C |
| Weakness | General body fatigue |
| Stomach Pain | Abdominal discomfort |
| Constipation | Irregular bowel movement |
| Headache | Persistent |
| Loss of Appetite | Reduced desire to eat |
Complications in Severe Cases
- Intestinal perforation — a life-threatening condition where the intestinal wall ruptures
- Can lead to death if untreated
Diagnosis
- Typhoid fever is confirmed by the Widal Test
- This test detects antibodies against Salmonella typhi in the patient's blood
Historic Case — "Typhoid Mary"
Mary Mallon — "Typhoid Mary"
| Detail | Information |
|---|---|
| Real Name | Mary Mallon |
| Nickname | Typhoid Mary |
| Profession | Cook |
| Condition | Typhoid carrier (infected but showed no symptoms) |
| Impact | Spread typhoid for several years through food she prepared |
Key Lesson from Typhoid Mary
A person can be a carrier of a disease — carrying and spreading the pathogen — without showing any symptoms themselves. This makes carriers particularly dangerous in disease transmission, as they remain undetected while continuously infecting others.
Summary Flow
Contaminated Food/Water
↓
Enters Small Intestine
↓
Salmonella typhi multiplies
↓
Migrates through Blood
↓
Affects Multiple Organs
↓
Symptoms: Fever, Pain, Weakness
↓
Severe Case → Intestinal Perforation → Death
Key Takeaway
Typhoid is a food and water-borne disease caused by Salmonella typhi, preventable through clean drinking water, proper sanitation, and safe food handling practices.
04
Pneumonia
Causative Organisms
Two bacteria are responsible for Pneumonia:
- Streptococcus pneumoniae
- Haemophilus influenzae
Target Organ & Mechanism of Damage
Organ Affected: Lungs — specifically the Alveoli (air-filled sacs)
Normally, alveoli are air-filled and responsible for oxygen-carbon dioxide exchange. When infected, they fill with fluid, severely compromising this vital function.
Bacteria infect the Alveoli
↓
Alveoli get filled with FLUID
↓
Gaseous exchange is disrupted
↓
Severe Respiratory Problems
Symptoms of Pneumonia
| Symptom | Severity |
|---|---|
| Fever | Common |
| Chills | Common |
| Cough | Common |
| Headache | Common |
| Lips & fingernails turning gray/bluish | Severe cases only |
Why Do Lips & Nails Turn Bluish? — Cyanosis
- Fluid-filled alveoli block oxygen exchange
- Reduced oxygen in blood (hypoxia)
- Blood becomes deoxygenated, giving a bluish tint to lips and nails
Mode of Transmission
A healthy person can acquire pneumonia through:
| Route | Description |
|---|---|
| Inhalation | Breathing in droplets/aerosols released by an infected person |
| Sharing utensils | Using common glasses and utensils with an infected person |
Other Bacterial Diseases in Humans
| Disease | Causative Bacterium |
|---|---|
| Dysentery | Shigella spp. |
| Plague | Yersinia pestis |
| Diphtheria | Corynebacterium diphtheriae |
Comparison: Typhoid vs Pneumonia
| Feature | Typhoid | Pneumonia |
|---|---|---|
| Pathogen | Salmonella typhi | Streptococcus pneumoniae |
| Target Organ | Small intestine / systemic | Lungs (Alveoli) |
| Transmission | Contaminated food & water | Droplets / shared utensils |
| Key Symptom | Sustained high fever | Respiratory distress |
| Severe Complication | Intestinal perforation | Cyanosis |
Key Takeaway
Pneumonia is a serious respiratory bacterial disease affecting the alveoli of the lungs. It spreads through airborne droplets and shared utensils, making respiratory hygiene and avoiding close contact with infected individuals the most important preventive measures.
05
The Common Cold
Causative Organism
| Feature | Detail |
|---|---|
| Type | Virus |
| Pathogen | Rhino viruses |
| Nature | One of the most infectious human ailments |
Target Organs
| Organ Affected | Organ Spared |
|---|---|
| ✅ Nose | ❌ Lungs |
| ✅ Respiratory Passage |
Important Distinction
Unlike pneumonia which affects the lungs (alveoli), the common cold is restricted to the upper respiratory tract.
Symptoms of Common Cold
| Symptom | Description |
|---|---|
| Nasal Congestion | Blocked/stuffy nose |
| Nasal Discharge | Runny nose |
| Sore Throat | Irritation & pain in throat |
| Hoarseness | Rough/raspy voice |
| Cough | Persistent irritation |
| Headache | General pain |
| Tiredness | Overall fatigue & weakness |
Duration
Symptoms usually last for 3 to 7 days.
Mode of Transmission
Route 1 — Direct Transmission
Infected Person Coughs/Sneezes
↓
Releases Droplets into Air
↓
Healthy Person Inhales Droplets
↓
Infection Occurs
Route 2 — Indirect Transmission (Fomites)
Infected Person touches Objects
↓
Objects become Contaminated
↓
Healthy Person touches same Objects
↓
Infection Transmitted
Common Contaminated Objects (Fomites)
| Object | Setting |
|---|---|
| Pens & Books | School / Office |
| Cups | Home / Cafeteria |
| Doorknobs | Public places |
| Computer Keyboard | Office / Home |
| Computer Mouse | Office / Home |
These objects are called Fomites — inanimate objects that can carry and transmit infection.
Comparison: Common Cold vs Pneumonia
| Feature | Common Cold | Pneumonia |
|---|---|---|
| Pathogen | Rhino virus | Streptococcus pneumoniae |
| Type | Viral | Bacterial |
| Organs Affected | Nose & Respiratory passage | Lungs (Alveoli) |
| Severity | Mild | Severe |
| Duration | 3–7 days | Longer, needs treatment |
| Transmission | Droplets & Fomites | Droplets & shared utensils |
| Fatal? | Rarely | Can be fatal |
Key Takeaway
The common cold, caused by Rhino viruses, is one of the most infectious but least fatal human diseases. Its spread through both airborne droplets and contaminated surfaces (fomites) makes it extremely common. Simple preventive habits like handwashing, avoiding touching the face, and covering the mouth while sneezing/coughing are the most effective ways to prevent its spread.
06
Malaria
Causative Organism
| Species | Type of Malaria | Severity |
|---|---|---|
| P. vivax | Benign tertian malaria | Moderate |
| P. malariae | Quartan malaria | Moderate |
| P. falciparum | Malignant malaria | Most serious / Fatal |
Hosts Involved
Plasmodium requires TWO hosts to complete its life cycle
| Host | Role |
|---|---|
| Human | Intermediate host — site of asexual reproduction |
| Female Anopheles mosquito | Primary host AND Vector (transmitting agent) |
Life Cycle of Plasmodium
Phase 1 — Entry into Human Body
Infected Female Anopheles mosquito bites Human
↓
Sporozoites (infectious form) injected into bloodstream
↓
Sporozoites travel through blood to the LIVER
Phase 2 — Multiplication in Liver
Sporozoites enter Liver Cells
↓
Asexual Reproduction occurs in Liver cells
↓
Liver cells BURST
↓
Parasites released into Blood
Phase 3 — Attack on Red Blood Cells (RBCs)
Parasites enter Red Blood Cells (RBCs)
↓
Asexual Reproduction in RBCs
↓
RBCs RUPTURE (burst)
↓
Release of toxic substance — HAEMOZOIN
↓
Chill and High Fever every 3–4 days
↓
Released parasites infect NEW RBCs (cycle repeats)
Phase 4 — Sexual Stage in RBCs
Some parasites develop into Male & Female Gametocytes in RBCs.
Phase 5 — Transfer back to Mosquito
Female Anopheles mosquito bites Infected Person
↓
Takes up Gametocytes with blood meal
↓
Fertilization & Development in Mosquito's GUT
↓
Sporozoites formed & stored in Salivary Glands
↓
Mosquito bites another Human → Cycle repeats
Key Substance — Haemozoin
| Feature | Detail |
|---|---|
| What is it? | A toxic substance released during RBC rupture |
| Released when? | When RBCs burst due to parasite reproduction |
| Effect | Causes chills and high fever |
| Frequency | Recurring every 3 to 4 days |
Key Terminology
| Term | Meaning |
|---|---|
| Sporozoites | Infectious form of Plasmodium injected by mosquito |
| Haemozoin | Toxic substance causing fever & chills |
| Vector | Transmitting agent — Female Anopheles mosquito |
| Gametocytes | Sexual stage of parasite developing in RBCs |
| Malignant Malaria | Most dangerous form caused by P. falciparum |
Key Takeaway
Malaria is a protozoan disease caused by Plasmodium, transmitted through the bite of infected female Anopheles mosquito. The disease involves a complex two-host life cycle and the rupture of RBCs releases haemozoin, causing the characteristic recurring fever. P. falciparum causes the most fatal form of malaria, making it one of the world's most dangerous infectious diseases.
07
Amoebiasis (Amoebic Dysentery)
Causative Organism
| Feature | Detail |
|---|---|
| Pathogen | Entamoeba histolytica |
| Type | Protozoan parasite |
| Site of Infection | Large intestine of humans |
| Disease Caused | Amoebiasis / Amoebic Dysentery |
Symptoms of Amoebiasis
| Symptom | Description |
|---|---|
| Constipation | Difficulty in passing stools |
| Abdominal Pain | Pain in the stomach region |
| Abdominal Cramps | Spasms in the abdominal area |
| Excess Mucous in Stools | Abnormal mucous discharge |
| Blood Clots in Stools | Sign of intestinal damage/bleeding |
Hallmark Symptom
The presence of blood and mucous in stools is the hallmark symptom that distinguishes amoebic dysentery from other intestinal disorders.
Mode of Transmission
Primary Sources of Infection
Faeces of Infected Person
↓
Contaminates Water & Food
↓
Consumed by Healthy Person
↓
Infection in Large Intestine
Role of Houseflies — Mechanical Carriers
Infected Person's Faeces
↓
Housefly sits on faeces
↓
Parasite sticks to fly's body
↓
Fly lands on Food/Food Products
↓
Food gets Contaminated
↓
Healthy Person consumes food → Amoebiasis
🪰 Mechanical Carrier
Houseflies do NOT get infected themselves but simply carry and transfer the parasite from faeces to food.
Comparison: Amoebiasis vs Typhoid
| Feature | Amoebiasis | Typhoid |
|---|---|---|
| Pathogen | Entamoeba histolytica | Salmonella typhi |
| Type | Protozoan | Bacterium |
| Organ Affected | Large intestine | Small intestine |
| Transmission | Contaminated food/water + Houseflies | Contaminated food/water |
| Key Symptom | Blood & mucous in stools | Sustained high fever |
| Carrier | Housefly (mechanical) | Human carrier (Typhoid Mary) |
Key Takeaway
Amoebiasis is a protozoan intestinal disease caused by Entamoeba histolytica, primarily spread through the faecal-oral route via contaminated food and water. Houseflies play a critical role as mechanical carriers, making food hygiene, safe drinking water, and proper sanitation the most essential preventive measures.
08
Ascariasis & Helminth Diseases
What are Helminths?
Helminths are parasitic worms that cause diseases in humans. They are multicellular organisms unlike bacteria, viruses, or protozoans. Two major pathogenic helminths:
| Helminth | Common Name | Disease Caused |
|---|---|---|
| Ascaris | Common Roundworm | Ascariasis |
| Wuchereria | Filarial Worm | Filariasis |
Ascariasis — Causative Organism
| Feature | Detail |
|---|---|
| Pathogen | Ascaris |
| Common Name | Roundworm |
| Type | Helminth (parasitic worm) |
| Site of Infection | Intestine (intestinal parasite) |
| Disease | Ascariasis |
Symptoms of Ascariasis
| Symptom | Explanation |
|---|---|
| Internal Bleeding | Worms damage intestinal walls causing bleeding |
| Muscular Pain | Larvae migrate through muscles |
| Fever | Immune response to parasitic infection |
| Anaemia | Blood loss due to internal bleeding |
| Blockage of Intestinal Passage | Heavy worm burden blocks intestinal lumen |
Most Dangerous Complication
The blockage of intestinal passage is the most dangerous complication and can be life-threatening if untreated.
Life Cycle & Mode of Transmission
Step 1 — Egg Excretion
Infected Person
↓
Ascaris eggs excreted with FAECES
↓
Eggs contaminate: Soil, Water, Plants
Step 2 — Transmission to Healthy Person
Contaminated Water, Vegetables, Fruits
↓
Consumed by Healthy Person
↓
Eggs enter digestive system → hatch into larvae
↓
Develop into adult worms → Ascariasis established
Comparison: Ascariasis vs Filariasis
| Feature | Ascariasis | Filariasis |
|---|---|---|
| Pathogen | Ascaris | Wuchereria |
| Common Name | Roundworm | Filarial worm |
| Transmission | Contaminated food/water | Mosquito bite |
| Organ Affected | Intestine | Lymphatic system |
| Key Symptom | Intestinal blockage, anaemia | Massive limb swelling |
| Vector | None (direct) | Mosquito |
Key Takeaway
Ascariasis is a helminthic intestinal disease caused by the roundworm Ascaris, transmitted through contaminated food, water, and soil. Its serious complications like internal bleeding, anaemia, and intestinal blockage make it a significant public health concern, especially in areas with poor sanitation.
09
Filariasis (Elephantiasis)
Causative Organism
| Feature | Detail |
|---|---|
| Pathogen | Wuchereria bancrofti and Wuchereria malayi |
| Common Name | Filarial Worms |
| Type | Helminth (parasitic worm) |
| Disease Caused | Filariasis / Elephantiasis |
Nature of Infection
Unlike most diseases — Filariasis is Chronic
- Slowly developing infection
- Chronic inflammation — long-term, persistent
- Worms live inside the host for many years
- Causes progressive and irreversible damage over time
Target Organs & Effects
| Organ | Effect |
|---|---|
| Lymphatic vessels of lower limbs (Primary) | Chronic inflammation → Blockage of lymph flow |
| Genital organs (Secondary) | Often affected → Gross deformities |
Why Does Swelling Occur?
Filarial Worms enter Lymphatic Vessels
↓
Live there for MANY YEARS
↓
Cause Chronic Inflammation
↓
Lymphatic vessels get BLOCKED
↓
Lymph fluid cannot drain properly
↓
Accumulates in Lower Limbs & Tissues
↓
MASSIVE SWELLING → Resembles Elephant's Leg → ELEPHANTIASIS
Symptoms of Filariasis
| Symptom | Description |
|---|---|
| Massive swelling of lower limbs | Due to lymph fluid accumulation |
| Thickening of skin | Skin becomes rough and hardened |
| Chronic inflammation | Long-term swelling of lymphatic vessels |
| Genital deformities | Gross deformities of reproductive organs |
| Fever | During acute phases |
| Pain & discomfort | In affected limbs |
Mode of Transmission
Infected Person carrying Filarial Worm larvae
↓
Female Mosquito bites infected person
↓
Takes up larvae (microfilariae) with blood meal
↓
Larvae develop inside mosquito
↓
Infected Mosquito bites Healthy Person
↓
Larvae migrate to Lymphatic vessels
↓
Develop into Adult Filarial Worms → Filariasis
Comparison: Filariasis vs Malaria
| Feature | Filariasis | Malaria |
|---|---|---|
| Pathogen | Wuchereria (Helminth) | Plasmodium (Protozoan) |
| Vector | Female Culex mosquito | Female Anopheles mosquito |
| Organ Affected | Lymphatic system | RBCs & Liver |
| Nature | Chronic, slow developing | Acute, recurring |
| Key Symptom | Massive limb swelling | Recurring fever & chills |
| Duration | Years | Weeks if untreated |
Key Takeaway
Filariasis (Elephantiasis) is a chronic helminthic disease caused by Wuchereria bancrofti and W. malayi, transmitted through female mosquito bites. The worms live in lymphatic vessels for years, causing progressive inflammation, blockage, and massive swelling of lower limbs and genital organs.
10
Ringworm (Fungal Disease)
Causative Organisms
| Fungi Genus | Target Area |
|---|---|
| Microsporum | Skin & Scalp |
| Trichophyton | Skin, Nails & Scalp |
| Epidermophyton | Skin & Nails |
Important Note
Ringworm is one of the most common infectious diseases in humans. Despite its name, it is caused by FUNGI, NOT worms.
Symptoms
| Symptom | Description |
|---|---|
| Dry, scaly lesions | Appear on skin, nails and scalp |
| Intense itching | Accompanies all lesions |
| Circular patches | Ring-like appearance on skin |
Body Parts Affected: Skin, Nails, Scalp
Conditions Favouring Fungal Growth
Heat + Moisture
↓
Ideal conditions for fungal growth
↓
Thrive in Skin Folds: Groin area, Between toes (Athlete's foot)
Mode of Transmission
| Source | Description |
|---|---|
| Soil | Fungi naturally present in soil |
| Towels | Shared towels of infected person |
| Clothes | Wearing clothes of infected person |
| Comb | Using comb of infected person |
Ringworm spreads through direct contact with infected persons or their personal belongings.
Key Takeaway
Ringworm is a fungal disease — NOT caused by worms. It thrives in warm, moist conditions and spreads through contact with infected persons or their personal items. Personal hygiene is the most effective preventive measure.
11
Prevention & Control
1. Personal Hygiene Measures
- Keeping the body clean
- Consumption of clean drinking water, food, vegetables, fruits
- Avoid sharing personal items (towels, combs, clothes)
- Wash hands before meals
2. Public Hygiene Measures
- Proper disposal of waste and excreta
- Periodic cleaning and disinfection of water reservoirs, pools, cesspools and tanks
- Observing standard hygiene practices in public catering
Particularly Essential For
These measures are especially important where infectious agents are transmitted through food and water — such as typhoid, amoebiasis and ascariasis.
3. Air-Borne Disease Prevention
For diseases such as pneumonia and common cold, in addition to hygiene measures:
- Avoid close contact with infected persons
- Avoid contact with their belongings
- Cover mouth while sneezing/coughing
4. Vector-Borne Disease Control
For diseases such as malaria and filariasis, the most important measure is to control or eliminate vectors and their breeding places:
- Avoiding stagnation of water in and around residential areas
- Regular cleaning of household coolers
- Use of mosquito nets
- Introducing fishes like Gambusia in ponds that feed on mosquito larvae
- Spraying of insecticides in ditches, drainage areas and swamps
- Doors and windows provided with wire mesh to prevent entry of mosquitoes
Gambusia Fish — Biological Control
| Feature | Detail |
|---|---|
| Name | Gambusia (Mosquitofish) |
| Role | Feeds on mosquito larvae |
| Where used | Introduced in ponds & water bodies |
| Type of control | Biological control method |
| Advantage | Eco-friendly, no chemicals needed |
Special Mention — Vector-Borne Diseases in India
These precautions have become more important especially in the light of recent widespread incidences of dengue and chikungunya (transmitted by Aedes mosquitoes) in many parts of India.
5. Vaccines & Immunisation
| Achievement | Disease |
|---|---|
| Complete Eradication | Smallpox |
| Largely Controlled | Polio |
| Largely Controlled | Diphtheria |
| Largely Controlled | Pneumonia |
| Largely Controlled | Tetanus |
6. Biotechnology & Future Vaccines
Biotechnology is at the verge of making available newer and safer vaccines. Discovery of antibiotics and various other drugs has also enabled us to effectively treat infectious diseases.
Key Takeaway
Prevention requires a multi-pronged approach combining personal hygiene, public sanitation, vector control, and medical interventions. The success of smallpox eradication through vaccination stands as the greatest achievement in medical history, while advances in biotechnology promise even more effective tools against infectious diseases in the future.
12
Complete Reference Table
All Diseases — Comparison
| Disease | Pathogen | Type | Organ Affected | Transmission | Key Symptom | Prevention |
|---|---|---|---|---|---|---|
| Typhoid | Salmonella typhi | Bacteria | Small intestine → systemic | Contaminated food/water | Sustained high fever (39–40°C) | Safe water, sanitation |
| Pneumonia | Streptococcus pneumoniae | Bacteria | Lungs (Alveoli) | Droplets, shared utensils | Breathing difficulty, cyanosis | Avoid contact, vaccine |
| Common Cold | Rhino viruses | Virus | Nose, respiratory passage | Droplets, fomites | Nasal congestion, sore throat | Avoid contact, hygiene |
| Malaria | Plasmodium spp. | Protozoan | Liver, RBCs | Female Anopheles mosquito | Recurring fever & chills (3–4 days) | Mosquito control |
| Amoebiasis | Entamoeba histolytica | Protozoan | Large intestine | Contaminated food/water, houseflies | Blood & mucous in stools | Safe water, sanitation |
| Ascariasis | Ascaris (Roundworm) | Helminth | Intestine | Contaminated food/water/soil | Intestinal blockage, anaemia | Hygiene, clean food |
| Filariasis | Wuchereria bancrofti | Helminth | Lymphatic vessels | Female Culex mosquito | Massive limb swelling | Mosquito control, DEC drug |
| Ringworm | Microsporum, Trichophyton | Fungi | Skin, nails, scalp | Soil, shared personal items | Dry scaly lesions, itching | Personal hygiene |
Vector Comparison
| Disease | Vector | Mosquito Species |
|---|---|---|
| Malaria | Female Anopheles | Anopheles spp. |
| Filariasis | Female Culex | Culex spp. |
| Dengue | Female Aedes | Aedes aegypti |
| Chikungunya | Female Aedes | Aedes aegypti |
★
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