Measuring disease in a community

Prevalence and incidence are connected by a rule that planners use constantly: in a steady state, prevalence is approximately incidence multiplied by the average length of the illness. If a disease occurs at 10 new cases per 1,000 people each year and lasts on average 5 years, roughly 50 people per 1,000 will have it at any moment — an illustrative calculation, not a measured figure. The rule exposes several traps. First, case fatality works like a wide drain: a disease that kills quickly shortens duration and therefore lowers prevalence, which is why a lethal infection can look rare in a clinic register. Second, a treatment that extends survival without curing — antiretroviral therapy is the standard example — widens the duration and pushes prevalence upward even while incidence falls. Third, screening detects prevalent cases, so it appears to discover mainly diseases of long duration; short illnesses that resolve or kill before the survey are missed, a distortion known as length bias. Epidemiologists therefore distinguish point prevalence, measured on a single date, from period prevalence, measured over an interval such as a year. Nursing uses both directions of the rule: incidence rates of pressure ulcers or surgical-site infections show whether prevention is working, while point-prevalence audits show how many patients currently need dressings, isolation or extra staffing.

A DALY is built from two components. The first, years of life lost (YLL), counts each death multiplied by the standard life expectancy remaining at the age when death occurred, so a death in childhood contributes far more than a death at eighty. The second, years lived with disability (YLD), multiplies the number of years a person lives with a condition by a disability weight between 0 and 1, where 0 is full health and 1 is equivalent to being dead. Adding YLL and YLD gives total DALYs, and one DALY always means one lost year of healthy life. That construction explains why burden of disease is not the same as cost of illness. Cost of illness adds direct medical expenditure and, ideally, indirect costs such as lost work, lost schooling, the support services a condition demands, and what people would pay to avoid the risk. Burden of disease measures suffering and premature death in healthy years. A vaccination programme may cost almost nothing per child yet prevent thousands of DALYs; a very expensive drug may relieve symptoms while leaving the burden nearly unchanged if it cannot restore function. WHO and the Global Burden of Disease study use DALYs precisely because money and suffering are different currencies, and a health ministry must budget in both. For nursing, the practical consequence is priority setting: a cheap, highly burdensome problem such as measles or diarrhoeal disease deserves a place beside costly, less burdensome ones.

Endemic and epidemic describe an observed count against an expected count, and the expected count comes from surveillance: the routine, continuous collection of case reports, laboratory results and deaths. A disease is endemic when that expected level is a constant presence — malaria, typhoid and influenza are the given examples — and epidemic when the observed count rises above the expected level, as cholera, diarrhoeal diseases and measles do. Several mechanisms move a disease from one state to the other. A growing susceptible pool does it: when vaccination coverage falls, measles, which had been controlled, finds enough non-immune children to spread. A new agent or a new route does it: contaminated water can turn a sporadic diarrhoeal illness into an explosive cholera epidemic within days. A change in the environment or in behaviour does it: crowding, displacement or a single contaminated meal can expose many people at once. Epidemiologists also separate a point-source epidemic, in which everyone is exposed at roughly the same moment, from a propagated epidemic, in which each case infects the next. An epidemic that spreads across countries becomes a pandemic. Duration matters here too: an acute epidemic such as cholera produces a sharp incidence peak that prevalence follows only briefly, whereas an epidemic of a chronic infection adds permanently to prevalence. Nurses are part of the detection system, because they often see and report the first unusual cluster of cases through the notifiable-disease channel, and they deliver the oral rehydration, isolation and hygiene measures that break the chain.