Curriculum · Public Health and Communicable Diseases
Incidence and prevalence
What it is
Prevalence is the patients who have the disease divided by all the population.
It comes in two forms the course files keep apart. Period prevalence is the proportion of a population who experienced the disease over a stated period, one year in the worked example. Point prevalence is the other, and the whole item turns on the two not being the same.
Incidence is the other half of the pair, and where both are given for one disease the units separate them. For infectious mononucleosis the epidemiology is written twice over: incidence in the US is 5:1000 population per year, and prevalence worldwide is more than 90% of the adult population EBV-antibody positive. The incidence value is set against a population and against a year; the prevalence value is a share of the population, with no year in it. So it is the units beside the number that say which of the two is being given.
How it is used
Prevalence is the statistic that answers how much disease a practice holds. Asked to estimate, from the statistics available for your area, the current number of patients with cancer you can expect to have in your community practice, the statistic that answers it is cancer prevalence, and not cancer incidence, relative risk of cancer, absolute cancer risk or cancer mortality.
The worked example runs the same way. Women aged 40 to 59 years attending their family practice setting were questioned, and 3.6% of them had had an eating disorder during the past year. With 100 women in that age group in your practice, and because the study applies to your clinic, about 4% of attendees are likely to experience an eating disorder in the coming year.
The second use is inside a diagnostic test: predictive values are affected with the prevalence. So the positive predictive value, which is how likely the patient has the disease, moves with how common the disease is in the population.