Incidence, Prevalence and Mortality Rates
On this page
Direct answer
Incidence measures new cases arising in a population at risk over a defined period, while prevalence measures all existing cases at a point (point prevalence) or over a period (period prevalence); the two are linked by the approximation prevalence equals incidence multiplied by average duration. Mortality rates then quantify deaths: the crude death rate uses the whole mid-year population, specific rates break it down by age, sex or cause, proportional mortality expresses a cause's share of all deaths, and the case fatality rate measures killing power among those already diseased. India's headline values — SRS 2020 infant mortality rate of 28 per 1000 live births and maternal mortality ratio of 97 per 100,000 live births (2018-20) — are quoted directly from these formulas.
What you must remember
- Incidence: new cases divided by population at risk over a time span; a rate, needing a time denominator — attack rate and person-time incidence are its variants.
- Prevalence: existing cases divided by total population at a point in time; a proportion, not a rate; rises when duration lengthens (better treatment of diabetes raises prevalence while lowering mortality).
- Core relation: prevalence = incidence × average duration; doubling survival of a disease doubles its point prevalence even if incidence is unchanged.
- Crude death rate: deaths per 1000 mid-year population; India's CDR is around 6-7 per 1000 — always attribute to source and year.
- Infant mortality rate: infant deaths (under one year) per 1000 live births in a year; India SRS 2020 = 28, with neonatal deaths now contributing roughly two-thirds of infant deaths.
- Maternal mortality ratio: maternal deaths per 100,000 live births; India = 97 (SRS 2018-20), with SDG target 70 by 2030.
- Case fatality rate: deaths from a disease divided by diagnosed cases of that disease — a disease-severity measure (rabies near 100 per cent), not a population-risk measure.
- Proportional mortality and PYLL: a cause's percentage of total deaths (lung cancer proportional mortality) and person-years of life lost emphasise premature mortality — PYLL, a favourite short-note in university papers.
Computing your way through a district
A district reports 2400 live births, 72 infant deaths, 600 existing tuberculosis cases of which 120 were diagnosed that year, in a mid-year population of 200,000. IMR = 72/2400 × 1000 = 30 per 1000. Tuberculosis incidence = 120/200,000 = 0.6 per 1000 per year; point prevalence = 600/200,000 = 3 per 1000 — five times the incidence, implying average duration of roughly five years, which the prevalence-equals-incidence-times-duration relation hands you as a sanity check. If 30 of the 600 tuberculosis patients died, case fatality rate = 5 per cent, and those deaths as a share of the district's 1400 total deaths give a proportional mortality just above 2 per cent.
Notice how each figure answers a different examiner's question. Incidence tells the epidemiologist about ongoing transmission; prevalence tells the planner how many treatment slots to fund; case fatality tells the clinician about prognosis; IMR and MMR tell the programme manager whether interventions are landing. This division of labour is a recurring five-mark theory question, and answers that show the decision attached to each number score above bare formula lists.
Where students slip
The classic errors are mechanical. Candidates call prevalence a rate; it is a proportion with no time denominator, and examiners deduct for it. They calculate case fatality rate using the population instead of cases — the 72 infant deaths above belong to a 2400-birth denominator, not to 200,000 people. They forget that IMR uses live births (stillbirths excluded, perinatal mortality is the rate that includes them) and that MMR is a ratio per 100,000 live births, not a proportion of pregnancies. A subtler viva trap: why does improved treatment of hypertension raise its prevalence — because duration lengthens while incidence stays put. Finally, always date and source every Indian statistic: SRS 2020 IMR 28 versus NFHS-5 (2019-21) IMR around 35 differ because survey and registry methods differ, and quoting either without its source invites the follow-up you cannot answer.
Frequently asked questions
What is the difference between incidence and prevalence?
Incidence counts new cases arising in a population at risk over a period, whereas prevalence counts all existing cases at a given point or period; incidence is a rate, prevalence is a proportion.
How are incidence and prevalence related?
Prevalence approximates incidence multiplied by the average duration of the disease, so chronic-disease prevalence rises when treatment prolongs survival.
What is the current infant mortality rate of India?
The SRS 2020 figure is 28 infant deaths per 1000 live births, with NFHS-5 (2019-21) reporting a somewhat higher survey-based estimate around 35.
How does case fatality rate differ from crude death rate?
Case fatality rate measures deaths among diagnosed cases of a specific disease, while crude death rate measures deaths from all causes in the entire mid-year population per 1000.
Why does India's maternal mortality use a ratio per 100,000 live births?
Because it relates maternal deaths to live births rather than to all pregnancies, and the per-100,000 scale expresses an uncommon event in whole, comparable numbers — 97 per 100,000 live births in SRS 2018-20.