Incidence & Prevalence: Calculating Prevalence

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In epidemiology we actually have three different ways to calculate the prevalence:

Point prevalence: the number of cases of a health event at a certain time. For example, in a survey you would be asked if you are currently smoking.
Period prevalence: the number of cases of a health event in reference to a time period, often 12 months. For example, in a survey you would be asked if you have smoked during the past 12 months.
Lifetime prevalence: the number of cases of the health event in reference to the total lifetime. For example, in a survey, you would be asked if you have ever smoked.