Learn — Concepts of health, disease & prevention

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Mosquito, ticks, mites, louse — identification and prevention

Arthropods of medical importance are studied under two headings in community health: which genus carries which disease, and what habits that genus has, because control is aimed at the habit and not at the insect in the abstract. It matters because vector-borne diseases account for a large share of India's communicable disease burden, and the nurse who can name where a vector breeds and when it bites can plan a control measure that works, while fogging and repellents alone never end an outbreak.

Anopheles mosquitoes transmit malaria; Anopheles culicifacies is the main rural vector in India and Anopheles stephensi the urban one. They breed in clean, still or slow-moving water such as paddy fields, borrow pits, wells and river margins, bite from dusk to dawn, rest indoors on walls after feeding, lay eggs singly with lateral floats, and the larvae lie parallel to the water surface while the adult rests at an angle to it.
Aedes aegypti transmits dengue, chikungunya, Zika and yellow fever. It is a black mosquito with white stripes and a lyre-shaped mark on the thorax, breeds in small collections of clean water in and around the house (desert coolers, overhead tanks, tyres, flower pots, discarded containers), bites by day with peaks in early morning and before dusk, has a flight range of only about 100 metres, and lays eggs on the container wall above the waterline that survive drying for months.
Culex quinquefasciatus transmits bancroftian filariasis and breeds in dirty organically polluted water such as drains, cesspits and soakage pits, biting at night; Culex tritaeniorhynchus transmits Japanese encephalitis, breeds in rice fields, and depends on pigs and ardeid birds as amplifying hosts. Culex lays eggs in rafts, its larvae hang head-down at an angle and the adult rests parallel to the surface.
Hard ticks transmit Indian tick typhus, Kyasanur Forest Disease (through Haemaphysalis species in the Western Ghats) and Crimean-Congo haemorrhagic fever, and can cause tick paralysis; because a tick usually has to feed for many hours before it transmits, prompt removal with steady traction close to the skin, without crushing or burning it, prevents infection.
Mites matter twice over: the larval trombiculid mite or chigger of Leptotrombidium transmits scrub typhus and leaves an eschar at the bite, while Sarcoptes scabiei is not a vector but the cause of scabies, burrowing into the finger web spaces and wrists and treated with permethrin or ivermectin along with the whole household on the same day.
The body louse Pediculus humanus corporis transmits epidemic typhus, louse-borne relapsing fever and trench fever, and is a disease of crowding, cold and unwashed clothing, so control is delousing of clothes and bedding by heat rather than treatment of the skin; the head louse is a nuisance and not a vector.
Other vectors examined regularly: the sandfly Phlebotomus argentipes transmits kala-azar and breeds in cracks in mud walls and cattle sheds with a short hop-flight, which is why indoor residual spraying is done up to about six feet on walls; the rat flea Xenopsylla cheopis transmits plague and murine typhus; and Cyclops, a freshwater copepod, carries guinea worm and fish tapeworm.
Control is layered: source reduction and the weekly dry day for Aedes, larvicides such as temephos in potable water and Bti in non-potable water, larvivorous fish (Gambusia and Poecilia) in tanks and ponds, indoor residual spraying for malaria and kala-azar, long-lasting insecticidal nets for night biters, environmental engineering, and personal protection with full sleeves and repellents; space spraying or fogging kills only flying adults during an epidemic and is never the mainstay.

Disease determinants

Determinants of disease are the factors that decide whether disease occurs at all, in whom it occurs and how severely, and they are grouped for teaching as agent, host and environment, the three points of the epidemiological triad. They matter because the point at which a programme intervenes is chosen from the triad, and in Indian community practice the environmental and social determinants are usually the weakest link and therefore the cheapest place to break the chain.

Agent factors are biological (bacteria, viruses, fungi, parasites), nutrient (deficiency of iron or iodine, or excess of fat and salt), physical (heat, cold, radiation, noise, humidity), chemical whether endogenous like urea and bilirubin or exogenous like pesticides, lead and fluoride, and mechanical (friction, trauma and chronic pressure), together with the absence of a needed factor such as insulin or a vitamin.
The agent is described in the field by measurable properties: infectivity, the ability to enter and multiply in the host; pathogenicity, the ability to produce clinical disease; virulence, the ability to produce severe disease and death; antigenicity, the ability to provoke immunity; and the infective dose required.
Host or intrinsic factors are age, sex, ethnic group, occupation, marital status, education, nutrition, immunity, heredity, habits and behaviour, and the physiological and psychological state; age and sex are the two host variables described in every descriptive study because they shape every disease pattern.
Environment or extrinsic factors are physical (air, water, housing, climate, waste disposal, radiation), biological (vectors, reservoirs, animals, plants and other humans) and psychosocial (culture, custom, income, education, occupation, social support and stress); the psychosocial environment is the part that community health nursing addresses through health education and community organisation.
The epidemiological triad explains infectious disease well; for chronic and non-communicable disease the model used is the web of causation or multifactorial causation, in which many interlocking chains produce the outcome and cutting any one strand reduces the disease even when the whole web is not understood, the classical illustration being myocardial infarction.
A risk factor is an attribute associated with an increased probability of disease and may be modifiable (smoking, diet, blood pressure, physical inactivity) or non-modifiable (age, sex, genetic make-up); a risk marker merely predicts and does not cause, and only a factor that satisfies temporality and the other Bradford Hill considerations is treated as causal.
The social determinants of health, namely income, education, employment, housing, food security, water and sanitation, gender and access to care, explain more of the difference in health between two Indian communities than medical care does, which is why safe water, latrines, girls' education and nutrition are counted as health interventions.
Prevention is applied at the level the determinant sits: primordial prevention stops the risk factor from ever entering the population, primary prevention removes the exposure or raises resistance through health promotion and specific protection, secondary prevention finds and treats disease early through screening and prompt treatment, and tertiary prevention limits disability and rehabilitates.

Pathogenicity vs virulence

Pathogenicity is the ability of an organism to produce clinical disease in those it has infected, while virulence is the ability of that organism to produce severe disease and death in those it has made ill. They are separate properties measured on different denominators, and confusing them is one of the commonest errors in the epidemiology section of the paper.

Infectivity is the number of people infected divided by the number of susceptible people exposed, expressed as a percentage; measles and chickenpox have close to 100 per cent infectivity in a susceptible household.
Pathogenicity is the number who develop clinical disease divided by the number infected; measles has very high pathogenicity because nearly everyone infected falls ill, while poliomyelitis has very low pathogenicity because most infections are inapparent, which is why the iceberg of polio is so deep and eradication needed mass campaigns rather than case-finding.
Virulence is the number of severe or fatal cases divided by the number of clinical cases, and the case fatality rate is its usual measure; rabies has effectively 100 per cent virulence once symptoms begin, while cholera is highly infective yet of low virulence wherever rehydration is available.
The secondary attack rate is the number of exposed susceptible contacts who develop the disease within one incubation period divided by the total susceptible contacts in that household or unit, with the index case excluded from both numerator and denominator; it measures communicability or infectivity in the field and is not a measure of pathogenicity.
The attack rate is the incidence measured over the short period of an outbreak, usually expressed per 100 exposed; the case fatality rate is deaths from the disease divided by cases of the disease and measures killing power, but it also reflects the quality and timeliness of the treatment given.
A worked example clarifies all three: of 100 susceptible people exposed, 50 become infected, so infectivity is 50 per cent; 30 of the 50 infected develop clinical illness, so pathogenicity is 60 per cent; and 3 of the 30 ill people die, so virulence measured as case fatality is 10 per cent.
These properties are described as belonging to the agent, but every value measured in the field also reflects the host and the care available, so a case fatality rate can fall sharply because treatment improved while the organism has not changed at all.

Incubation period table

The incubation period is the interval between the entry of an agent into the host and the appearance of the first sign or symptom of the disease. It matters in the field because it fixes the quarantine period for contacts, the period of surveillance after an exposure, the shape of the epidemic curve and, worked backwards from the peak of cases, the date and source of the exposure in an outbreak investigation.

Hours to a few days: staphylococcal food poisoning 1 to 6 hours, the emetic form of Bacillus cereus 1 to 6 hours and the diarrhoeal form 6 to 16 hours, Clostridium perfringens 6 to 24 hours, salmonella food poisoning 6 to 48 hours, cholera a few hours to 5 days and usually 1 to 2 days, influenza 1 to 3 days and diphtheria 2 to 5 days.
About one to two weeks: pertussis 7 to 14 days, poliomyelitis 7 to 14 days, dengue 3 to 10 days and usually 4 to 7, typhoid 10 to 14 days with a range of 3 to 21, malaria about 12 days for Plasmodium falciparum and about 14 for Plasmodium vivax, and measles about 10 days to the onset of fever and 14 days to the rash.
Two to three weeks: chickenpox 14 to 16 days with a range of 10 to 21, rubella 14 to 21 days, mumps about 18 days with a range of 12 to 26, and tetanus 6 to 10 days with a range of 3 to 21 days.
Long incubation: hepatitis A 15 to 50 days averaging about 28, hepatitis E 15 to 60 days averaging about 40, hepatitis B 30 to 180 days averaging 60 to 90, rabies commonly 3 to 8 weeks but ranging from days to years and shortest with bites on the face and other richly innervated parts, leprosy about 3 to 5 years for tuberculoid disease and much longer for lepromatous, and HIV to AIDS about 8 to 10 years without treatment.
Quarantine of a healthy exposed contact runs for the maximum incubation period of the disease counted from the last exposure, while isolation of a case runs for the period of communicability; the two words are not interchangeable and the distinction is examined every year.
The generation time or serial interval, which is the interval between receiving the infection and the point of maximum infectivity, matters more than the incubation period for transmission, because measles, chickenpox and pertussis are all infectious before the diagnostic sign appears.
An epidemic in which all the cases appear within one incubation period around a single sharp peak is a point-source or common-source single-exposure epidemic, whereas a curve with successive peaks separated by about one incubation period indicates propagated person-to-person spread; a common-source continuous exposure gives a plateau.
To find the exposure in an outbreak, subtract the median incubation period from the date of onset of the peak of cases, which is how a contaminated meal, water source or ceremony is identified even before any laboratory result arrives.

Types of prevention (primary/secondary/tertiary)

The levels of prevention are the four points at which the natural history of a disease, as described by Leavell and Clark, can be interrupted: before the risk factor appears, before the disease appears, before it declares itself clinically, and before it disables. Naming the level correctly is what decides the action, because health education, immunisation, screening and rehabilitation are not interchangeable and each belongs to a different stage of the same disease.

Primordial prevention, described by Strasser, prevents the emergence of risk factors in populations where they have not yet appeared, and its main tool is education and public policy; keeping schoolchildren from ever starting tobacco, or protecting a community's traditional diet before obesity arrives, is primordial.
Primary prevention acts in the pre-pathogenesis phase, before the disease process begins, and has two modes: health promotion (health education, nutrition, safe environment, lifestyle, adequate housing) and specific protection (immunisation, iodised salt, iron and folic acid supplements, chemoprophylaxis, use of personal protective equipment, chlorination of drinking water).
Secondary prevention acts in early pathogenesis through early diagnosis and prompt treatment; it halts progression, prevents complications, shortens the period of infectivity and, in communicable disease, protects the community as well as the patient. All screening is secondary prevention.
Tertiary prevention acts when the disease has advanced beyond its early stages, and has two modes: disability limitation and rehabilitation, the latter with medical, vocational, social, psychological and economic components.
The disability sequence to quote is disease leading to impairment (loss of structure or function), then to disability (restriction of activity) and then to handicap (the social disadvantage), which the WHO's later classification restates as impairment, activity limitation and participation restriction.
Indian programme examples anchor each level: peer education under RKSK is primordial; BCG at birth and iron and folic acid under Anemia Mukt Bharat are primary; RBSK screening of children from birth to eighteen years for the four Ds, CBAC-based screening of every adult over thirty at an Ayushman Arogya Mandir and sputum testing of a chest symptomatic under NTEP are secondary; and self-care groups, reconstructive surgery and disability prevention for leprosy under the national programme are tertiary.
Primary prevention is cheaper, safer and more effective than secondary, and secondary than tertiary; a programme forced to choose puts its money at the lowest level it can, which is why immunisation and safe water outrank hospital beds in a public health budget.

Iceberg phenomenon

The iceberg phenomenon of disease is the observation that the clinically apparent cases a health worker sees are only the small tip of a much larger submerged mass of undiagnosed, subclinical, latent and carrier states, with the biochemical and immunological changes that precede them below even that. It matters because the size and the danger of a health problem cannot be judged from an outpatient register: the hidden portion is where transmission continues, where complications are being built, and where screening and surveillance have to work.

The waterline is the level of clinical detection: above it are diagnosed clinical cases, below it lie undiagnosed cases, presymptomatic and subclinical disease, latent infection, carriers and, at the base, the sub-cellular changes that have not yet produced any illness.
Epidemiologically the submerged part matters more than the tip, because carriers and subclinical cases move about freely, are unaware and untreated, and keep transmission going; poliomyelitis is the classic example, in which inapparent infections vastly outnumber the paralytic cases that are seen.
Other communicable examples are hepatitis A and B, rubella, diphtheria carriers, typhoid carriers and latent tuberculosis infection, which is why the NTEP now offers TB preventive treatment to household contacts instead of waiting for them to cough.
Non-communicable disease has the largest iceberg of all: hypertension, diabetes, chronic kidney disease, cervical precancer and depression are silent for years, and the ICMR-INDIAB survey published in 2023 estimated diabetes in 11.4 per cent and prediabetes in 15.3 per cent of Indian adults, most of them undiagnosed.
The submerged part is measured, not guessed: community-based screening surveys, sero-epidemiological surveys, sentinel surveillance and active case finding reveal it, while hospital and outpatient data cannot, because they are selected by who chose to come.
The programme response is built on this idea: active case finding among vulnerable populations under NTEP, CBAC-based enumeration and screening of everyone over thirty at Ayushman Arogya Mandirs, HIV sentinel surveillance and testing in antenatal clinics, and RBSK screening of every child from birth to eighteen years.
The ratio of clinical to subclinical disease varies by agent and is a property of the disease, not of the health system; a disease with a large submerged portion needs screening and surveillance, while one that always declares itself clinically can be controlled by treating the cases who present.

Maslow hierarchy of needs

Maslow's hierarchy of needs, published by Abraham Maslow in 1943, arranges human motivation into five levels — physiological, safety and security, love and belonging, esteem, and self-actualisation — and holds that a lower need must be reasonably satisfied before a higher one drives behaviour. Nursing uses it as the rule for ordering care when several problems compete, which is why almost every question that asks which action the nurse should take first can be settled by finding the lowest unmet need.

Level one, physiological: oxygen, water, food, elimination, sleep, shelter, warmth and sexual expression; within this level the nurse orders by airway, breathing and circulation first, then pain, then elimination, nutrition and rest.
Level two, safety and security: physical safety such as fall prevention, infection control, safe medication practice and a clean water supply, and psychological security such as a consistent explanation, an interpreter and informed consent.
Level three, love and belonging: family, friendship and acceptance — letting a mother stay with her admitted child, involving the husband in counselling, linking a widow to a self-help group.
Level four, esteem: self-respect, competence, independence and recognition — teaching a diabetic to inject his own insulin, letting a colostomy patient do her own bag change, getting a recovered stroke patient back to earning.
Level five, self-actualisation: realising one's potential, problem solving, acceptance of reality and creativity; it is the last need addressed and the one most often neglected in a busy service.
The first four are deficiency needs, which weaken once satisfied, while self-actualisation is a growth need that strengthens the more it is met; Maslow later added cognitive, aesthetic and transcendence needs above esteem, but Indian nursing examinations use the five-level pyramid.
The hierarchy is a guide and not a law: an immediate threat to life or safety, such as an actively suicidal patient, a fire or a woman being beaten at home, is dealt with before a routine physiological need, and needs at different levels are commonly worked on at the same time.
In community practice the hierarchy explains failure: a family with no food, no safe water and no secure shelter will not act on advice about screening or family planning, so the nurse who first secures food, water and safety earns the standing to give the health education that follows.

Anthropology

Anthropology is the study of human beings — their biology and evolution on one side, and on the other the cultures they live in, with their beliefs, customs, kinship and food habits. Medical anthropology is the branch that asks how culture decides who is thought to be ill, whom the family calls for help and what treatment is accepted, and it is the discipline that explains why a technically perfect health programme fails in one village and succeeds in the next.

Physical or biological anthropology deals with human evolution, genetics, growth and anthropometry, and it is the source of the body measurements the nurse uses every day — weight, height, mid-upper arm circumference and head circumference.
Cultural and social anthropology deals with the customs, rituals, taboos, kinship and food habits of a community, and it explains the hot and cold classification of foods in pregnancy, the discarding of colostrum as impure, prelacteal feeds of honey or ghutti, the period of confinement after birth and the seclusion of the menstruating woman.
Medical anthropology distinguishes disease, the biomedical diagnosis, from illness, the patient's own experience of it, and from sickness, the social role the community gives him; a family may accept that a child is ill while rejecting the nurse's account of why, and the health worker must address both.
Culture is learned, shared, transmitted between generations and slow to change, so the community health nurse works with it rather than against it, using the opinion leader, the traditional birth attendant, the healer and the religious head as change agents.
The terms to be able to define are ethnocentrism, which is judging another culture by the standards of one's own and is the commonest error in health education; cultural relativism; acculturation; cultural lag; and the folk theories of causation such as the evil eye or the anger of a deity that compete with the germ theory.
Practical application in the field is a cultural assessment recorded in the family folder: who takes the decisions in the household, what the food taboos are for a pregnant woman and a young child, which language and dialect the message must be in, whether a woman may be examined by a male worker, and when the family is free of agricultural work to attend a session.

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