CBSE • Class XII • Biol
ogy
CBSE Board Class XII Biology
Human Health & Disease
Deeply Explained Chapter Notes — NCERT Biology, Class XII, CBSE
Immunity · Diseases · AIDS & Cancer · Drug & Alcohol Abuse — with mechanisms, reasoning & exam tips
1. Health & Disease — the Big Picture
Health (WHO definition) is a state of complete physical, mental and social well-being — not simply "not being sick." This is why a person can be free
of disease yet still be considered unhealthy if they are constantly stressed, malnourished, or socially isolated. Good health lets the body perform its
normal functions efficiently and lets a person cope with everyday physical and mental demands.
What determines whether we stay healthy?
Genetic factors — disorders inherited from parents (e.g. haemophilia, sickle-cell anaemia) that we are born with and cannot "catch."
Infections — caused by external pathogens (bacteria, viruses, fungi, protozoans, helminths) that invade the body.
Lifestyle — diet, exercise, rest, and habits like smoking or alcohol use, which is why balanced nutrition, regular exercise, adequate sleep, and
personal hygiene are emphasised as the first line of defence against disease, even before we talk about medicines.
Congenital Acquired
Present since birth — e.g. colour blindness, cleft lip. These arise Develop after birth, and are split into two kinds: Communicable
from developmental or genetic errors during foetal growth, not (caused by pathogens and capable of spreading from person to
from catching anything after birth. person — e.g. common cold) and Non-communicable (cannot
spread — e.g. cancer, hypertension, arthritis; usually caused by
internal malfunction, lifestyle, or long-term exposure).
✎ Why this distinction matters: public health strategy for communicable diseases focuses on breaking the chain of transmission (sanitation, vector
control, isolation), while for non-communicable diseases it focuses on lifestyle change and early screening.
✦ ✦ ✦
2. Common Diseases in Humans
Diseases are best understood by asking four questions each time: what causes it, where in the body it acts, how it produces symptoms, and how it
spreads. Below, each disease is broken down the same way.
Bacterial Diseases
Typhoid is caused by the rod-shaped bacterium Salmonella typhi. After being swallowed with contaminated food or water, the bacteria travel to the
small intestine, enter the bloodstream, and multiply — producing a sustained high fever (39–40°C) that classically climbs in a step-ladder pattern,
along with weakness, stomach pain, constipation, headache, and loss of appetite. Rose-coloured spots may appear on the skin. Diagnosis is confirmed
by the Widal test, which detects antibodies against Salmonella antigens in the patient's blood serum.
Pneumonia is caused by bacteria such as Streptococcus pneumoniae and Haemophilus influenzae. These infect the alveoli (tiny air sacs of the
lungs), which become inflamed and fill with fluid — this is why breathing becomes difficult: the fluid blocks the normal exchange of oxygen and
carbon dioxide. Symptoms include fever, chills, cough, and headache; in severe cases the lack of oxygen turns the lips and fingernails a grey-to-bluish
colour (cyanosis).
Viral Diseases
Common Cold is caused by Rhinoviruses, one of the smallest and most infectious viruses known. Unlike pneumonia, it infects only the nose and
upper respiratory passage — not the lungs — which is why it stays a "cold" rather than becoming serious. It spreads through droplets released while
coughing/sneezing, and through contact with contaminated surfaces (door handles, utensils). Because it is caused by a virus (not bacteria),
antibiotics do not help; the infection is self-limiting and usually clears in 3–7 days as the immune system's interferons and antibody response take over.
Protozoan Diseases
, Malaria is caused by species of the protozoan Plasmodium — P. vivax, P. malariae, and P. falciparum (the most dangerous, responsible for the most
severe/fatal cases as it can cause cerebral malaria). The disease is spread by the bite of a female Anopheles mosquito (only females bite, since they
need a blood meal to develop their eggs).
Life Cycle — step by step
1. An infected female Anopheles mosquito injects sporozoites (the infective stage of the parasite) into the human bloodstream while feeding.
2. Sporozoites travel to the liver, where they multiply asexually inside liver cells (this "silent" phase explains the incubation period before
symptoms begin).
3. The parasites are released from the liver and invade red blood cells (RBCs), where they multiply further.
4. Infected RBCs eventually burst, releasing more parasites and a toxic pigment called haemozoin into the bloodstream. It is this sudden
release of haemozoin that triggers the characteristic chills, high fever, and sweating.
5. Because RBCs burst in synchronised waves, the fever recurs periodically — roughly every 3–4 days, depending on the species (this
periodicity is a classic diagnostic clue).
6. Some parasites develop into sexual forms called gametocytes inside RBCs. When another mosquito bites the infected person, it picks up
these gametocytes, and they develop further inside the mosquito's gut — completing the cycle and allowing transmission to a new host.
Amoebiasis (amoebic dysentery) is caused by Entamoeba histolytica, a single-celled parasite that infects the large intestine. It causes constipation,
abdominal pain and cramps, and stools containing excess mucus and blood clots (because the parasite erodes the intestinal lining). It spreads through
food or water contaminated with faecal matter, and houseflies act as mechanical carriers, picking up cysts on their bodies and depositing them on
food.
Helminthic (Worm) Diseases
Ascariasis is caused by Ascaris, a large intestinal roundworm. Eggs passed in human faeces contaminate soil, water, and vegetables grown in that
soil; when swallowed, they hatch and the larvae migrate through the body before settling in the intestine. Heavy infestations cause internal bleeding,
muscular pain, fever, anaemia (due to nutrient loss to the worms), and even blockage of the intestinal passage in severe cases.
Filariasis (Elephantiasis) is caused by the filarial worms Wuchereria bancrofti and W. malayi, transmitted by the bite of a female Culex
mosquito. The thread-like worms lodge in the lymphatic vessels, particularly of the lower limbs and reproductive organs, causing chronic
inflammation. Over years, this blocks lymph drainage and causes the classic gross swelling/deformation of the limbs that gives the disease its common
name.
Fungal Diseases
Ringworm is caused by fungi of the genera Microsporum, Trichophyton, and Epidermophyton. These fungi thrive on keratin, so they infect the skin,
nails, and scalp, producing dry, scaly lesions with intense itching. Infection spreads through contact with soil, an infected person or animal, or shared
towels/clothing, and is aggravated by heat and moisture (which is why it is more common in hot, humid climates and among people who sweat
heavily or wear damp clothing for long periods).
Disease Pathogen Type Spread / Site
Typhoid Salmonella typhi Bacteria Food/water → intestine → blood
Pneumonia Streptococcus pneumoniae Bacteria Droplet → alveoli
Common Cold Rhinovirus Virus Droplet/contact → nose & upper respiratory tract
Malaria Plasmodium spp. Protozoan Female Anopheles → liver & RBCs
Amoebiasis Entamoeba histolytica Protozoan Food/water → large intestine
Ascariasis Ascaris Helminth Food/water/soil → intestine
Filariasis Wuchereria spp. Helminth Female Culex → lymphatic vessels
Ringworm Microsporum, etc. Fungus Contact → skin/nails/scalp
fever in malaria is due to synchronised RBC rupture , not
Exam Tip: Vector-borne diseases — Malaria (Anopheles), Filariasis (Culex). The periodic
the mosquito bite itself.
ogy
CBSE Board Class XII Biology
Human Health & Disease
Deeply Explained Chapter Notes — NCERT Biology, Class XII, CBSE
Immunity · Diseases · AIDS & Cancer · Drug & Alcohol Abuse — with mechanisms, reasoning & exam tips
1. Health & Disease — the Big Picture
Health (WHO definition) is a state of complete physical, mental and social well-being — not simply "not being sick." This is why a person can be free
of disease yet still be considered unhealthy if they are constantly stressed, malnourished, or socially isolated. Good health lets the body perform its
normal functions efficiently and lets a person cope with everyday physical and mental demands.
What determines whether we stay healthy?
Genetic factors — disorders inherited from parents (e.g. haemophilia, sickle-cell anaemia) that we are born with and cannot "catch."
Infections — caused by external pathogens (bacteria, viruses, fungi, protozoans, helminths) that invade the body.
Lifestyle — diet, exercise, rest, and habits like smoking or alcohol use, which is why balanced nutrition, regular exercise, adequate sleep, and
personal hygiene are emphasised as the first line of defence against disease, even before we talk about medicines.
Congenital Acquired
Present since birth — e.g. colour blindness, cleft lip. These arise Develop after birth, and are split into two kinds: Communicable
from developmental or genetic errors during foetal growth, not (caused by pathogens and capable of spreading from person to
from catching anything after birth. person — e.g. common cold) and Non-communicable (cannot
spread — e.g. cancer, hypertension, arthritis; usually caused by
internal malfunction, lifestyle, or long-term exposure).
✎ Why this distinction matters: public health strategy for communicable diseases focuses on breaking the chain of transmission (sanitation, vector
control, isolation), while for non-communicable diseases it focuses on lifestyle change and early screening.
✦ ✦ ✦
2. Common Diseases in Humans
Diseases are best understood by asking four questions each time: what causes it, where in the body it acts, how it produces symptoms, and how it
spreads. Below, each disease is broken down the same way.
Bacterial Diseases
Typhoid is caused by the rod-shaped bacterium Salmonella typhi. After being swallowed with contaminated food or water, the bacteria travel to the
small intestine, enter the bloodstream, and multiply — producing a sustained high fever (39–40°C) that classically climbs in a step-ladder pattern,
along with weakness, stomach pain, constipation, headache, and loss of appetite. Rose-coloured spots may appear on the skin. Diagnosis is confirmed
by the Widal test, which detects antibodies against Salmonella antigens in the patient's blood serum.
Pneumonia is caused by bacteria such as Streptococcus pneumoniae and Haemophilus influenzae. These infect the alveoli (tiny air sacs of the
lungs), which become inflamed and fill with fluid — this is why breathing becomes difficult: the fluid blocks the normal exchange of oxygen and
carbon dioxide. Symptoms include fever, chills, cough, and headache; in severe cases the lack of oxygen turns the lips and fingernails a grey-to-bluish
colour (cyanosis).
Viral Diseases
Common Cold is caused by Rhinoviruses, one of the smallest and most infectious viruses known. Unlike pneumonia, it infects only the nose and
upper respiratory passage — not the lungs — which is why it stays a "cold" rather than becoming serious. It spreads through droplets released while
coughing/sneezing, and through contact with contaminated surfaces (door handles, utensils). Because it is caused by a virus (not bacteria),
antibiotics do not help; the infection is self-limiting and usually clears in 3–7 days as the immune system's interferons and antibody response take over.
Protozoan Diseases
, Malaria is caused by species of the protozoan Plasmodium — P. vivax, P. malariae, and P. falciparum (the most dangerous, responsible for the most
severe/fatal cases as it can cause cerebral malaria). The disease is spread by the bite of a female Anopheles mosquito (only females bite, since they
need a blood meal to develop their eggs).
Life Cycle — step by step
1. An infected female Anopheles mosquito injects sporozoites (the infective stage of the parasite) into the human bloodstream while feeding.
2. Sporozoites travel to the liver, where they multiply asexually inside liver cells (this "silent" phase explains the incubation period before
symptoms begin).
3. The parasites are released from the liver and invade red blood cells (RBCs), where they multiply further.
4. Infected RBCs eventually burst, releasing more parasites and a toxic pigment called haemozoin into the bloodstream. It is this sudden
release of haemozoin that triggers the characteristic chills, high fever, and sweating.
5. Because RBCs burst in synchronised waves, the fever recurs periodically — roughly every 3–4 days, depending on the species (this
periodicity is a classic diagnostic clue).
6. Some parasites develop into sexual forms called gametocytes inside RBCs. When another mosquito bites the infected person, it picks up
these gametocytes, and they develop further inside the mosquito's gut — completing the cycle and allowing transmission to a new host.
Amoebiasis (amoebic dysentery) is caused by Entamoeba histolytica, a single-celled parasite that infects the large intestine. It causes constipation,
abdominal pain and cramps, and stools containing excess mucus and blood clots (because the parasite erodes the intestinal lining). It spreads through
food or water contaminated with faecal matter, and houseflies act as mechanical carriers, picking up cysts on their bodies and depositing them on
food.
Helminthic (Worm) Diseases
Ascariasis is caused by Ascaris, a large intestinal roundworm. Eggs passed in human faeces contaminate soil, water, and vegetables grown in that
soil; when swallowed, they hatch and the larvae migrate through the body before settling in the intestine. Heavy infestations cause internal bleeding,
muscular pain, fever, anaemia (due to nutrient loss to the worms), and even blockage of the intestinal passage in severe cases.
Filariasis (Elephantiasis) is caused by the filarial worms Wuchereria bancrofti and W. malayi, transmitted by the bite of a female Culex
mosquito. The thread-like worms lodge in the lymphatic vessels, particularly of the lower limbs and reproductive organs, causing chronic
inflammation. Over years, this blocks lymph drainage and causes the classic gross swelling/deformation of the limbs that gives the disease its common
name.
Fungal Diseases
Ringworm is caused by fungi of the genera Microsporum, Trichophyton, and Epidermophyton. These fungi thrive on keratin, so they infect the skin,
nails, and scalp, producing dry, scaly lesions with intense itching. Infection spreads through contact with soil, an infected person or animal, or shared
towels/clothing, and is aggravated by heat and moisture (which is why it is more common in hot, humid climates and among people who sweat
heavily or wear damp clothing for long periods).
Disease Pathogen Type Spread / Site
Typhoid Salmonella typhi Bacteria Food/water → intestine → blood
Pneumonia Streptococcus pneumoniae Bacteria Droplet → alveoli
Common Cold Rhinovirus Virus Droplet/contact → nose & upper respiratory tract
Malaria Plasmodium spp. Protozoan Female Anopheles → liver & RBCs
Amoebiasis Entamoeba histolytica Protozoan Food/water → large intestine
Ascariasis Ascaris Helminth Food/water/soil → intestine
Filariasis Wuchereria spp. Helminth Female Culex → lymphatic vessels
Ringworm Microsporum, etc. Fungus Contact → skin/nails/scalp
fever in malaria is due to synchronised RBC rupture , not
Exam Tip: Vector-borne diseases — Malaria (Anopheles), Filariasis (Culex). The periodic
the mosquito bite itself.
