Human Population and Health Disorders

This unit turns the biology of a single human body outwards onto whole populations. First it asks how many of us there are, how fast that number is changing, and what shape the population has — because the age structure of Nepal decides how many schools, jobs and hospital beds the country will need. Then it asks what makes people ill: microbes that pass from person to person, disorders that grow out of the way we live, and diseases caused simply by something missing from the plate.

9.1 Human Population

Demography is the statistical study of human populations — their size, composition, distribution and the rates at which they change. It rests on three processes only: births add people, deaths remove them, and migration moves them between places.

9.1.1 The basic measures

Every demographic rate is a count divided by the population at risk, multiplied by a convenient factor (1000 or 100,000) so that the answer is a whole-ish number.

where B is the number of live births in the year, D the number of deaths and P the mid-year population. Two more measures matter in Nepal:

• Maternal mortality ratio (MMR) — maternal deaths per 100,000 live births.
• Sex ratio — in Nepal this is reported as males per 100 females.

9.1.2 How populations grow

If a population grows by a fixed number each year the growth is arithmetic and the graph is a straight line. Real populations grow by a fixed proportion each year, so the increase itself gets bigger every year. This is exponential (geometric) growth, and its graph is the familiar J-shaped curve:

Here P_0 is the population at the start, P_t the population after time t years, and r the annual growth rate expressed as a decimal fraction.

Nepal has been counted every ten years since 1911, so its growth curve is one of the best-documented in South Asia.

9.1.3 Age–sex structure and the population pyramid

A population pyramid (age–sex pyramid) is a pair of back-to-back horizontal bar graphs: age groups stacked from youngest at the bottom to oldest at the top, males to the left of the axis and females to the right. Its shape is a fingerprint of a country's birth rate, death rate and migration history.

Pyramid shape	Birth rate	Death rate	Growth	Example
Expanding — broad base, narrow top	high	high	rapid	Niger, Afghanistan
Stable — nearly vertical sides	moderate	low	slow	Nepal today, India
Declining — narrow base, wide middle	low	low	negative	Japan, Italy

Nepal's own age structure has changed visibly in one decade. The base of the pyramid is now narrower than the band above it — the 0–9 group (5.21 million) is smaller than the 10–19 group (5.88 million). That is the arithmetic signature of falling fertility.

The dependency ratio turns the age structure into a single number: how many people who are not of working age each 100 workers must support.

9.1.4 The demographic transition

The demographic transition model (DTM) describes the path every industrialising society has followed from high birth and death rates to low ones. It has four recognised stages (a fifth, of population decline, is sometimes added).

Stage	Birth rate	Death rate	Natural increase	Why
1 High stationary	high (~40)	high (~40)	near zero	famine, epidemics, no medicine
2 Early expanding	high	falls steeply	very rapid	clean water, vaccines, better food
3 Late expanding	falls	low	slows down	contraception, girls' education, urban life
4 Low stationary	low (~12)	low (~10)	near zero	small planned families

Nepal moved through Stage 2 between about 1950 and 1990 — malaria control in the Terai, smallpox eradication and expanded immunisation cut deaths while births stayed high, which is why growth peaked at 2.62 % in the 1970s. Since 1990 the birth rate has fallen sharply: the total fertility rate was 4.6 children per woman in 1996 and 2.1 in 2022 (Nepal Demographic and Health Survey 2022) — essentially replacement level.

9.1.5 Nepal's population situation

Indicator	Value	Year and source
Total population	29,164,578	Census 2021 (NSO)
Male : female	14,253,551 : 14,911,027	Census 2021
Sex ratio	95.59 males per 100 females	Census 2021
Annual growth rate	0.92 %	Census 2011–2021
Population density	198 per km²	Census 2021
Households / mean size	6,666,937 / 4.37 persons	Census 2021
Urban (municipality) share	66.17 %	Census 2021
Absent population abroad	2,190,592 (82.2 % male)	Census 2021
Literacy (age 5+)	76.2 % (M 83.6, F 69.4)	Census 2021
Life expectancy at birth	71.3 yr (M 68.2, F 73.8)	Census 2021 (NSO)
Total fertility rate	2.1 children per woman	NDHS 2022
Infant mortality rate	28 per 1000 live births	NDHS 2022
Neonatal mortality rate	21 per 1000 live births	NDHS 2022
Under-five mortality rate	33 per 1000 live births	NDHS 2022
Maternal mortality ratio	151 per 100,000 live births	NMMS 2021
Projected population	30,034,040 (1 July 2026)	NSO projection

Four features of this table deserve comment.

1. There are more women than men. Nepal is one of few countries with a sex ratio below 100, and the reason is on the next line of the table: over 2.1 million Nepalis, four-fifths of them men, were abroad on census day.
2. Growth has almost stopped. Falling fertility plus emigration gave the slowest growth in 110 years of census-taking.
3. Nepal has become an urban country on paper. The 66 % urban share follows the 2015–17 redrawing of local units, which converted many rural areas into municipalities; much of that "urban" land is still farmland.
4. Mortality has fallen fast. Maternal deaths fell from 539 per 100,000 live births in 1996 to 151 in 2021 — one of the fastest declines recorded anywhere.

9.2 Health disorders

A disease is any disturbance of the normal structure or working of the body that produces recognisable signs and symptoms. Diseases are grouped as follows.

• Communicable (infectious) — caused by a pathogen and transmissible from one host to another: bacterial, viral, protozoan, helminthic, fungal.
• Non-communicable (NCD) — not transmissible: degenerative, metabolic, neoplastic (cancer), allergic and genetic disorders.
• Deficiency diseases — caused by lack of a nutrient.

9.2.1 Communicable diseases

Disease	Pathogen	Transmission	Main symptoms	Prevention
Tuberculosis	Mycobacterium tuberculosis (bacterium)	airborne droplets	chronic cough >2 weeks, blood in sputum, evening fever, weight loss	BCG vaccine, DOTS treatment, ventilation
Typhoid	Salmonella typhi (bacterium)	faecal contamination of food and water	step-ladder fever, rose spots, abdominal pain	safe water, TCV vaccine, hygiene
Cholera	Vibrio cholerae (bacterium)	contaminated water	painless rice-water stools, rapid dehydration	chlorination, ORS, oral vaccine
Tetanus	Clostridium tetani (bacterium)	spores entering deep wounds	lock-jaw, muscle spasm, opisthotonus	TT/DPT vaccine, wound cleaning
Pneumonia	Streptococcus pneumoniae and others	droplets	fast breathing, chest indrawing, fever	PCV vaccine, avoid indoor smoke
Influenza	Influenza A/B virus	droplets	fever, body ache, sore throat	annual vaccine, cough etiquette
Hepatitis B	Hepatitis B virus	blood, sexual contact, mother to child	jaundice, dark urine, liver damage	HBV vaccine, screened blood
Rabies	Rabies virus (Lyssavirus)	bite or lick of an infected dog, bat, jackal	hydrophobia, excitement, paralysis, death	dog vaccination, post-exposure vaccine
Dengue	Dengue virus (4 serotypes)	bite of Aedes aegypti / Ae. albopictus	high fever, retro-orbital pain, rash, joint pain	destroy clean-water breeding sites
AIDS	Human immunodeficiency virus	unsafe sex, infected blood/needles, mother to child	opportunistic infections, wasting	safe sex, screened blood, ART
Malaria	Plasmodium spp. (protozoan)	bite of female Anopheles	cold–hot–sweating fever paroxysms, anaemia, enlarged spleen	bed nets, spraying, prompt treatment
Amoebic dysentery	Entamoeba histolytica (protozoan)	cysts in food and water	stool with blood and mucus, cramps	safe water, hand washing
Kala-azar	Leishmania donovani (protozoan)	bite of Phlebotomus argentipes sandfly	long fever, darkened skin, huge spleen	indoor spraying, plastered walls
Ascariasis	Ascaris lumbricoides (roundworm)	eggs in soil-contaminated food	abdominal pain, malnutrition, worms in stool	latrine use, deworming, washing vegetables
Taeniasis	Taenia solium (tapeworm)	undercooked pork with cysticerci	abdominal discomfort, segments in stool	meat inspection, thorough cooking
Ringworm	Trichophyton, Microsporum (fungi)	contact, shared towels and combs	itchy ring-shaped scaly patches	keep skin dry, do not share clothing

How the marker reads a disease answer

Four marks usually means four facts: pathogen, mode of transmission, two or three symptoms, and one control measure. Write the pathogen's scientific name in italics with a capital for the genus and lower case for the species — Plasmodium vivax, not plasmodium Vivax. Markers do deduct for this.

9.2.2 Malaria — the parasite and its two hosts

Malaria is caused by four species of Plasmodium: P. vivax (benign tertian malaria, fever every 48 h), P. falciparum (malignant tertian, 48 h, the species that kills), P. malariae (quartan, 72 h) and P. ovale. In Nepal P. vivax is by far the commonest. The vector is the female Anopheles mosquito; the recognised vectors in Nepal are Anopheles fluviatilis, An. annularis, An. maculatus and An. minimus.

The fever comes from the bursting of red cells. In P. vivax the erythrocytic cycle takes 48 hours, so paroxysms recur every third day (counting inclusively) — hence "tertian". Each paroxysm has three phases: a cold stage with shivering, a hot stage with high fever, and a sweating stage as the temperature falls.

9.2.3 AIDS and HIV

AIDS — acquired immunodeficiency syndrome — is the late stage of infection with HIV, a retrovirus of the family Retroviridae. HIV is a spherical enveloped virus about 100–120 nm across, with two identical strands of single-stranded RNA and three enzymes of its own.

The target of HIV is the helper T lymphocyte (CD4⁺ T cell), the cell that coordinates the whole immune response. Reverse transcriptase copies viral RNA into DNA, integrase splices that DNA into the host chromosome as a provirus, and the infected cell then manufactures new virions until it is destroyed.

HIV spreads by unprotected sexual contact, transfusion of infected blood or sharing of needles, and from mother to child across the placenta, during birth or in breast milk. It does not spread by touching, sharing food, mosquito bites, coughing or using the same toilet. There is no cure, but antiretroviral therapy (ART) holds the virus down for decades.

9.2.4 Immunity and the immune response

Immunity is the ability of the body to resist a pathogen.

Basis	Type	Example
Present from birth, non-specific	innate (natural) immunity	skin, mucus, lysozyme in tears, stomach acid, phagocytes, inflammation
Developed after exposure, specific	acquired (adaptive) immunity	antibodies after chickenpox
Body makes its own antibodies	active — natural (after infection) or artificial (vaccine)	BCG, DPT, measles–rubella
Ready-made antibodies given	passive — natural (placenta, colostrum) or artificial (antiserum)	anti-tetanus serum, anti-rabies serum

An antigen is any foreign molecule that provokes a response; an antibody (immunoglobulin) is a Y-shaped protein made by plasma cells that binds one antigen specifically. The five classes are IgG (most abundant, the only one crossing the placenta), IgM (first made, largest), IgA (in mucus, saliva and colostrum), IgE (allergy) and IgD (receptor on B cells).

Because the helper T cell sits at the centre of the diagram, destroying it — as HIV does — disables both limbs at once. That is why AIDS patients die of ordinary organisms: tuberculosis, Pneumocystis pneumonia, candidiasis, Kaposi's sarcoma.

A vaccine is a preparation of killed, weakened (attenuated), toxoid or subunit antigen that produces active artificial immunity together with memory cells, without causing the disease.

9.2.5 Non-communicable diseases

Disease	Nature	Main causes / risk factors	Prevention
Hypertension	persistent BP ≥ 140/90 mm Hg	salt, obesity, alcohol, stress, heredity	less salt, exercise, weight control
Coronary heart disease	narrowing of coronary arteries by atheroma	smoking, high LDL, diabetes, inactivity	no tobacco, diet, exercise
Stroke	brain artery blocked or ruptured	hypertension, smoking, atrial fibrillation	blood-pressure control
COPD	irreversible airway obstruction	tobacco smoke, biomass and indoor smoke, dust	clean cooking fuel, no smoking
Asthma	reversible bronchospasm	allergens, cold air, exercise, pollution	avoid triggers, inhalers
Diabetes mellitus	type 1 — no insulin; type 2 — insulin resistance	heredity, obesity, inactivity, sugar-rich diet	weight control, exercise
Cancer	uncontrolled cell division, metastasis	tobacco, chewing tobacco, HPV, radiation, aflatoxin	screening, vaccination, no tobacco
Goitre	enlarged thyroid	dietary iodine deficiency	iodised salt
Arthritis	joint inflammation / cartilage wear	age, autoimmunity, obesity	weight control, physiotherapy
Allergy	hypersensitivity, IgE-mediated	pollen, dust mite, foods, drugs	avoid allergen, antihistamines

9.2.6 Deficiency diseases

Missing nutrient	Disease	Chief signs
Protein + energy	kwashiorkor	oedema of feet, moon face, flaky skin, enlarged liver
Energy (severe)	marasmus	extreme wasting, "old man" face, no subcutaneous fat
Vitamin A	night blindness → xerophthalmia	poor vision in dim light, Bitot's spots, corneal ulcer
Vitamin B₁ (thiamine)	beriberi	leg oedema (wet) or nerve damage and weakness (dry)
Vitamin B₂ (riboflavin)	cheilosis	cracks at corners of the mouth, sore red tongue
Vitamin B₃ (niacin)	pellagra	the three D's — dermatitis, diarrhoea, dementia
Vitamin B₁₂ / folate	megaloblastic anaemia	large immature red cells, weakness, sore tongue
Vitamin C	scurvy	bleeding spongy gums, loose teeth, slow wound healing
Vitamin D	rickets (child), osteomalacia (adult)	bow legs, pigeon chest, soft painful bones
Vitamin K	bleeding tendency	prolonged clotting time
Iron	iron-deficiency anaemia	pallor, tiredness, breathlessness, spoon nails
Iodine	goitre; cretinism in infants	neck swelling; stunted growth and mental retardation
Calcium	tetany, osteoporosis	muscle cramps, brittle bones

Nepal's own figures show real progress with real gaps. The Nepal Demographic and Health Survey 2022 found 25 % of children under five stunted (down from 57 % in 1996), 8 % wasted and 19 % underweight, while 43 % of children aged 6–59 months and 34 % of women aged 15–49 were anaemic. Universal salt iodisation has made endemic goitre in the hills, once very common, rare.

Chapter summary

• \text{CBR} = (B/P)\times 1000; \text{CDR} = (D/P)\times 1000; \text{RNI} = \text{CBR}-\text{CDR}; IMR uses live births as denominator.
• Exponential growth: P_t = P_0e^{rt}, so r = \frac{1}{t}\ln(P_t/P_0) and doubling time T_d = 0.693/r \approx 70/R\%.
• Dependency ratio = \dfrac{P_{0-14}+P_{65+}}{P_{15-64}}\times 100; for Nepal in 2021 it is 53.3 (child 42.7, old-age 10.6).
• Census 2021: 29,164,578 people, growth 0.92 % per year, sex ratio 95.59, density 198 km⁻², 66.17 % urban, 2.19 million absent abroad, life expectancy 71.3 years. NDHS 2022: TFR 2.1, IMR 28, U5MR 33. NMMS 2021: MMR 151.
• Population pyramids are expanding (broad base), stable or declining (narrow base); Nepal's base is now narrower than the band above it.
• The demographic transition runs from Stage 1 (high birth, high death) through Stage 2 (death falls, growth fastest) and Stage 3 (birth falls) to Stage 4 (both low). Nepal is in late Stage 3.
• Malaria: human = intermediate host (schizogony in liver and RBC); female Anopheles = definitive host (gamogony and sporogony); infection by sporozoites in saliva.
• HIV is a retrovirus that destroys CD4⁺ helper T cells; AIDS is diagnosed when the CD4⁺ count falls below 200 cells mm⁻³.
• Immunity is innate or acquired; acquired immunity is active (infection or vaccine) or passive (placenta, colostrum, antiserum), and has a humoral (antibody) and a cell-mediated limb.

Practice questions

Group A — Multiple choice (1 mark each)

1. The definitive host of the malarial parasite is [1]amanbfemale Anophelescmale AnophelesdCulex
2. Crude death rate is expressed as deaths per [1]a100 populationb1000 populationc1000 live birthsd100,000 live births
3. AIDS is diagnosed when the CD4⁺ T-cell count falls below [1]a1000 mm⁻³b800 mm⁻³c500 mm⁻³d200 mm⁻³
4. A population pyramid with a broad base and a narrow apex indicates [1]aa declining populationba stationary populationca rapidly growing populationdzero growth
5. Which immunoglobulin crosses the placenta? [1]aIgAbIgMcIgGdIgE
6. The annual population growth rate reported by Nepal's 2021 census was [1]a0.92 %b1.35 %c2.25 %d2.62 %

Answers to Group A

1. (b) — fertilisation and sporogony occur in the female Anopheles.

2. (b) — CDR is per 1000 mid-year population; (c) and (d) are IMR and MMR.

3. (d) — below 200 cells mm⁻³ the patient is defined as having AIDS.

4. (c) — many children and few old people means high birth rate and fast growth.

5. (c) — IgG is the only class small enough to cross the placenta, giving the newborn passive natural immunity.

6. (a) — 0.92 %, the lowest positive rate since censuses began in 1911.

Group B — Short answer (4 marks each)

1. Define crude birth rate, crude death rate and rate of natural increase. A district of 480,000 people registered 8160 live births and 3360 deaths in one year; find all three rates. [4]
2. What is a population pyramid? Draw and label the pyramid of a rapidly growing population and state two features that identify it. [4]
3. Explain the four stages of the demographic transition model and state, with a reason, which stage Nepal is in. [4]
4. Differentiate between communicable and non-communicable diseases, giving two examples of each. [4]
5. The 2021 census counted 29,164,578 people in Nepal's area of 147,516 km². Calculate the population density, and the population expected in 2031 if growth continues at 0.92 % per year. [4]
6. Write short notes on active and passive immunity, with one example of each. [4]

Answers to Group B

1. Definitions as in §9.1.1. \text{CBR} = \frac{8160}{480000}\times 1000 = 17.0 per 1000; \text{CDR} = \frac{3360}{480000}\times 1000 = 7.0 per 1000; \text{RNI} = 17.0 - 7.0 = 10.0 per 1000, i.e. 1.0 % per year (doubling time \approx 70 years).

3. Outline: Stage 1 both rates high, growth near zero; Stage 2 death rate falls (clean water, vaccines) while birth rate stays high, growth fastest; Stage 3 birth rate falls (contraception, female education), growth slows; Stage 4 both rates low, growth near zero. Nepal is in late Stage 3 — CDR is already low (about 6 per 1000) and the birth rate has fallen to about 19 per 1000 with TFR 2.1, so natural increase is small but still positive.

5. Density = \dfrac{29\,164\,578}{147\,516} = 197.7 \approx 198 persons km⁻².

P_{2031} = 29\,164\,578\times e^{0.0092\times 10} = 29\,164\,578\times 1.0964 = 3.20\times 10^{7}

About 32.0 million — an increase of roughly 2.8 million in ten years.

Group C — Long answer (8 marks each)

1. (a) Describe the life cycle of Plasmodium vivax in man and in the mosquito with a labelled diagram. [6](b) Name the vector, state why the mosquito is called the definitive host, and give two control measures used in Nepal. [2]
2. (a) Draw a labelled diagram of the HIV virion and describe how HIV infects a helper T cell. [4](b) List the modes of transmission of HIV, describe the three clinical phases of untreated infection, and state four preventive measures. [4]
3. The 2011 census recorded 26,494,504 people and the 2021 census 29,164,578, an interval of 10.43 years. The 2021 age distribution was 0–14: 8,115,575; 15–64: 19,027,289; 65+: 2,021,714.(a) Calculate the annual growth rate and the doubling time. [4](b) Calculate the total, child and old-age dependency ratios and explain what they tell a planner about Nepal's future. [4]

Answer outline to Group C question 3

(a) P_t/P_0 = 1.1008, \ln 1.1008 = 0.0960, so r = 0.0960/10.43 = 0.00921 = 0.92\ \% per year, and T_d = 0.693/0.00921 = 75.2 years.

(b) Total dependency = \frac{10\,137\,289}{19\,027\,289}\times 100 = 53.3; child = 42.7; old-age = 10.6. Interpretation: two-thirds of Nepalis are of working age, so the country is inside its demographic dividend and needs jobs and skills above all. But old-age dependency is rising (65+ rose from 5.27 % of the population in 2011 to 6.93 % in 2021), so pensions, geriatric care and NCD services must be built now, before the dividend closes.