Human Health, Diseases & Immunity

• Health is a state of complete physical, mental and social well-being, not merely absence of disease (WHO definition).
• Disease (dis-ease) is any condition where the body's normal working is disturbed, shown by symptoms like fever, pain or weakness.
• Infectious (communicable) diseases are caused by a germ and spread person-to-person, e.g. cholera, TB, AIDS.
• Non-infectious (non-communicable) diseases do not spread; they arise from inside the body, e.g. cancer, diabetes, scurvy.
• The germ that actually causes an infectious disease is called a pathogen.
• Normal human body temperature is 37 degrees C (98.6 degrees F).

Health is a state of complete physical, mental and social well-being, not merely absence of disease (WHO definition).

Disease (dis-ease) is any condition where the body's normal working is disturbed, shown by symptoms like fever, pain or weakness.

Infectious (communicable) diseases are caused by a germ and spread person-to-person, e.g. cholera, TB, AIDS.

Non-infectious (non-communicable) diseases do not spread; they arise from inside the body, e.g. cancer, diabetes, scurvy.

The germ that actually causes an infectious disease is called a pathogen.

• The five main pathogen groups are viruses, bacteria, fungi, protozoa and worms (helminths).
• Speed rule: viruses, bacteria and fungi multiply very fast, while worms multiply very slowly.
• All viruses live only inside host cells; bacteria can live and multiply outside cells.
• Key trap: antibiotics work against bacteria but NOT against viruses, because viruses lack the life processes antibiotics block.
• Virus — Common cold, flu, dengue, polio, AIDS, hepatitis-B
• Bacteria — Typhoid, cholera, TB, tetanus, anthrax, diphtheria
• Fungi — Ringworm, athlete's foot, late blight of potato
• Protozoa — Malaria (Plasmodium), kala-azar
• Worms — Elephantiasis (filariasis), taeniasis (tapeworm)

All viruses live only inside host cells; bacteria can live and multiply outside cells.

• A virus is the smallest germ, so small it passes through a bacteria-proof filter.
• It is genetic material (either DNA or RNA, never both) wrapped in a protein coat called the capsid, made of subunits called capsomeres.
• A virus is an obligate parasite (a nucleoprotein): alive only inside a host, inert outside.
• The name virus was given by Dmitri Ivanowsky (1892) while studying tobacco mosaic disease.
• M.W. Beijerinck (1898) called it Contagium vivum fluidum; W.M. Stanley (1935) crystallised it.
• A bacteriophage infects bacteria; the Hershey-Chase experiment used it to prove DNA is the genetic material.

A virus is the smallest germ, so small it passes through a bacteria-proof filter.

It is genetic material (either DNA or RNA, never both) wrapped in a protein coat called the capsid, made of subunits called capsomeres.

A virus is an obligate parasite (a nucleoprotein): alive only inside a host, inert outside.

M.W. Beijerinck (1898) called it Contagium vivum fluidum; W.M. Stanley (1935) crystallised it.

A bacteriophage infects bacteria; the Hershey-Chase experiment used it to prove DNA is the genetic material.

• Airborne spread: germs ride in droplets from coughs/sneezes, e.g. common cold, pneumonia, TB.
• Waterborne spread: germs enter via dirty water/food, e.g. typhoid and cholera (classic waterborne diseases).
• Vector-borne spread: an insect carrier (vector) ferries the germ; the mosquito is the most important vector.
• Direct/sexual contact: STIs like syphilis, gonorrhoea and AIDS spread this way.
• AIDS can also spread via shared needles, infected blood transfusion, or mother-to-foetus through the placenta.
• Air (droplet) — Common cold, pneumonia, TB
• Water/food — Typhoid, cholera
• Vector (insect) — Malaria, dengue, filariasis
• Sexual/blood — Syphilis, gonorrhoea, AIDS

Airborne spread: germs ride in droplets from coughs/sneezes, e.g. common cold, pneumonia, TB.

Waterborne spread: germs enter via dirty water/food, e.g. typhoid and cholera (classic waterborne diseases).

Vector-borne spread: an insect carrier (vector) ferries the germ; the mosquito is the most important vector.

Direct/sexual contact: STIs like syphilis, gonorrhoea and AIDS spread this way.

• The female Anopheles mosquito spreads malaria, and also filariasis and encephalitis.
• The Aedes (tiger) mosquito spreads dengue and chikungunya.
• The Culex mosquito spreads filariasis and Japanese encephalitis.
• Resting posture clue: Anopheles sits at an angle (tilted); Culex sits parallel (flat) to the surface.
• Anopheles (female) — Malaria, filariasis, encephalitis
• Aedes — Dengue, chikungunya, yellow fever, Zika
• Culex — Filariasis, Japanese encephalitis

The female Anopheles mosquito spreads malaria, and also filariasis and encephalitis.

The Aedes (tiger) mosquito spreads dengue and chikungunya.

The Culex mosquito spreads filariasis and Japanese encephalitis.

Resting posture clue: Anopheles sits at an angle (tilted); Culex sits parallel (flat) to the surface.

• Pneumonia is an infection of the lungs; air sacs fill with fluid causing cough and breathlessness.
• Anthrax (caused by Bacillus anthracis) is called splenic fever; it affects cattle and humans.
• Taeniasis is intestinal tapeworm infection (Taenia solium, saginata, asiatica); cysts in brain cause neurocysticercosis.
• Malaria (Plasmodium, via female Anopheles) is treated with chloroquine/quinine; quinine comes from Cinchona bark.
• Tetanus (lockjaw) causes muscle stiffness; treated with antitoxin (ATS) and prevented by vaccine.
• Haemophilus influenzae bacterium causes respiratory illness/meningitis; citrus canker is a bacterial plant disease.

Pneumonia is an infection of the lungs; air sacs fill with fluid causing cough and breathlessness.

Anthrax (caused by Bacillus anthracis) is called splenic fever; it affects cattle and humans.

Taeniasis is intestinal tapeworm infection (Taenia solium, saginata, asiatica); cysts in brain cause neurocysticercosis.

Malaria (Plasmodium, via female Anopheles) is treated with chloroquine/quinine; quinine comes from Cinchona bark.

Tetanus (lockjaw) causes muscle stiffness; treated with antitoxin (ATS) and prevented by vaccine.

Haemophilus influenzae bacterium causes respiratory illness/meningitis; citrus canker is a bacterial plant disease.

• Tuberculosis (TB) is prevented by the BCG vaccine.
• Diphtheria is covered by the DPT vaccine (diphtheria, pertussis, tetanus).
• Tetanus is prevented by vaccine and treated with antitetanus serum (ATS / antitoxin).
• Malaria is treated with chloroquine / quinine.
• TB — BCG vaccine
• Diphtheria — DPT vaccine
• Tetanus — Vaccine + ATS antitoxin
• Malaria — Chloroquine / quinine
• Polio — Oral polio vaccine (OPV)

Tuberculosis (TB) is prevented by the BCG vaccine.

Diphtheria is covered by the DPT vaccine (diphtheria, pertussis, tetanus).

Tetanus is prevented by vaccine and treated with antitetanus serum (ATS / antitoxin).

Malaria is treated with chloroquine / quinine.

• AIDS = Acquired Immuno-Deficiency Syndrome, caused by the Human Immunodeficiency Virus (HIV).
• HIV is a retrovirus with an RNA genome inside an envelope.
• It copies RNA into DNA using the enzyme reverse transcriptase ('retro' = backwards).
• HIV first enters macrophages, which become an HIV factory, then attacks helper T-lymphocytes.
• Destroying helper T-cells weakens immunity, so the body falls prey to other infections.
• It spreads by unprotected sex, infected blood/needles, and mother-to-child; not by casual touch.

AIDS = Acquired Immuno-Deficiency Syndrome, caused by the Human Immunodeficiency Virus (HIV).

HIV is a retrovirus with an RNA genome inside an envelope.

It copies RNA into DNA using the enzyme reverse transcriptase ('retro' = backwards).

HIV first enters macrophages, which become an HIV factory, then attacks helper T-lymphocytes.

Destroying helper T-cells weakens immunity, so the body falls prey to other infections.

• Immunity is the body's defence; the most important cells are the white blood cells (WBCs / leucocytes).
• Lymphocytes are the key immune cells: B-cells make antibodies, T-cells direct cell-mediated defence.
• An antigen is a foreign substance; the body makes matching antibodies to neutralise it.
• Lymphocytes are formed in the bone marrow (T-cells mature in the thymus).
• Innate immunity is present from birth; acquired immunity develops after exposure or vaccination.
• Leukaemia is blood cancer with an abnormal increase in white blood cells.

Immunity is the body's defence; the most important cells are the white blood cells (WBCs / leucocytes).

Lymphocytes are the key immune cells: B-cells make antibodies, T-cells direct cell-mediated defence.

An antigen is a foreign substance; the body makes matching antibodies to neutralise it.

Lymphocytes are formed in the bone marrow (T-cells mature in the thymus).

Innate immunity is present from birth; acquired immunity develops after exposure or vaccination.

Leukaemia is blood cancer with an abnormal increase in white blood cells.

• Antibodies are protein molecules called immunoglobulins (Ig) produced by B-lymphocytes.
• IgM is the first antibody produced in response to a new infection.
• IgG is the most abundant and provides long-term protection; it crosses the placenta.
• IgA guards mucous membranes (saliva, tears, milk); IgE is linked to allergies.
• IgM — First produced in infection
• IgG — Most abundant, long-term, crosses placenta
• IgA — Protects mucosa (saliva, tears, milk)
• IgE — Allergy and parasite response

Antibodies are protein molecules called immunoglobulins (Ig) produced by B-lymphocytes.

IgG is the most abundant and provides long-term protection; it crosses the placenta.

IgA guards mucous membranes (saliva, tears, milk); IgE is linked to allergies.

• A vaccine introduces a harmless form of a pathogen (an antigen) so the body makes antibodies in advance.
• This builds memory cells, giving fast protection if the real germ attacks later.
• Live attenuated vaccines use weakened germs; inactivated vaccines use killed germs.
• mRNA vaccines deliver genetic instructions so cells make the viral spike protein (e.g. Pfizer, Moderna against Covid-19).
• Vector vaccines use a harmless carrier virus to deliver the antigen gene (e.g. Covishield).
• Covishield is made by the Serum Institute of India (Oxford-AstraZeneca technology).

A vaccine introduces a harmless form of a pathogen (an antigen) so the body makes antibodies in advance.

This builds memory cells, giving fast protection if the real germ attacks later.

Live attenuated vaccines use weakened germs; inactivated vaccines use killed germs.

mRNA vaccines deliver genetic instructions so cells make the viral spike protein (e.g. Pfizer, Moderna against Covid-19).

Vector vaccines use a harmless carrier virus to deliver the antigen gene (e.g. Covishield).

Covishield is made by the Serum Institute of India (Oxford-AstraZeneca technology).

• Step 1: The vaccine introduces an antigen recognised as foreign.
• Step 2: B-lymphocytes produce antibodies against that antigen.
• Step 3: The body forms memory cells that remember the pathogen.
• Step 4: On real exposure, memory cells trigger a fast, strong secondary response.

Step 1: The vaccine introduces an antigen recognised as foreign.

Step 2: B-lymphocytes produce antibodies against that antigen.

Step 3: The body forms memory cells that remember the pathogen.

Step 4: On real exposure, memory cells trigger a fast, strong secondary response.

• Scurvy (bleeding gums) is due to lack of Vitamin C.
• Beri-beri is caused by lack of thiamine (Vitamin B1); eating only polished rice causes it.
• Kwashiorkor and marasmus are due to protein deficiency in children.
• Iron deficiency causes anaemia; iodine deficiency causes goitre.
• Dengue fever causes a sharp fall in platelets (thrombocytes).
• Vitamin C — Scurvy
• Vitamin B1 (thiamine) — Beri-beri
• Protein — Kwashiorkor, marasmus
• Iron — Anaemia
• Iodine — Goitre

Scurvy (bleeding gums) is due to lack of Vitamin C.

Beri-beri is caused by lack of thiamine (Vitamin B1); eating only polished rice causes it.

Kwashiorkor and marasmus are due to protein deficiency in children.

Dengue fever causes a sharp fall in platelets (thrombocytes).

• Cancer is uncontrolled, abnormal cell division forming tumours.
• Oncogenes are cancer-causing genes; normal genes can mutate into them.
• Carcinogens (cancer-causing agents) include tobacco, UV rays and certain chemicals; UV rays cause skin cancer.
• Radiation can both cause cancer and, in controlled doses, treat it (radiotherapy).
• Leukaemia is cancer of white blood cells (blood cancer).
• Lifestyle/non-communicable diseases include diabetes, hypertension and heart disease.

Cancer is uncontrolled, abnormal cell division forming tumours.

Oncogenes are cancer-causing genes; normal genes can mutate into them.

Carcinogens (cancer-causing agents) include tobacco, UV rays and certain chemicals; UV rays cause skin cancer.

Radiation can both cause cancer and, in controlled doses, treat it (radiotherapy).

Leukaemia is cancer of white blood cells (blood cancer).

• Epidemic: disease spreads rapidly across a large community or region.
• Pandemic: an outbreak spreads across many countries or the whole world (e.g. Covid-19).
• Endemic: a disease constantly present in a particular area.
• Insomnia is a sleep disorder (inability to sleep); sleep apnea is repeated stopping of breathing during sleep.
• Anorexia is loss/lack of appetite; bulimia involves not knowing when to stop eating.
• Dyspnoea means shortness of breath; anosmia means loss of the sense of smell.
• Epidemic — Spreads across a region
• Pandemic — Spreads worldwide
• Endemic — Always present in an area

Epidemic: disease spreads rapidly across a large community or region.

Pandemic: an outbreak spreads across many countries or the whole world (e.g. Covid-19).

Endemic: a disease constantly present in a particular area.

Insomnia is a sleep disorder (inability to sleep); sleep apnea is repeated stopping of breathing during sleep.

Anorexia is loss/lack of appetite; bulimia involves not knowing when to stop eating.

Dyspnoea means shortness of breath; anosmia means loss of the sense of smell.

• Refrigeration preserves food by slowing the growth of microbes, not by killing them.
• Pasteurisation heats milk to kill harmful germs while keeping it usable.
• Clean water and sanitation prevent waterborne diseases like typhoid and cholera.
• Vaccination and good ventilation cut airborne and preventable infections.

Refrigeration preserves food by slowing the growth of microbes, not by killing them.

Pasteurisation heats milk to kill harmful germs while keeping it usable.

Clean water and sanitation prevent waterborne diseases like typhoid and cholera.

Vaccination and good ventilation cut airborne and preventable infections.