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Incidence refers to the frequency of development of a new illness in a population in a certain period of time, normally one year. When we say that the incidence of a cancer has increased in past years, we mean that more people have developed this condition year after year, i.e.:, the incidence of thyroid cancer has been rising, with 13,000 new cases diagnosed this year.

Prevalence refers to the current number of people suffering from an illness in a given year. This number includes all those who may have been diagnosed in prior years, as well as in the current year. The incidence of a cancer is 20,000 year with a prevalence of 80,000 means that there are 20,000 new cases diagnosed every year and there are 80,000 people living with this illness, 60,000 of whom were diagnosed in the past decade and are still living with the disease. The number of people cured of the disease is not included in prevalence.