1 For best results from your flask, before use, pre-fill for about 3 minutes, using either hot water for hot drinks or cold water for chilled drinks. 2  Never use a microwave or conventional oven to pre-heat your flask.  If your flask has not been used for some time, rinse with warm water to remove any residual odours.
2. : the degree to which something is prevalent; especially: the percentage of a population that is affected with a particular disease at a given time.
Prevalence is expressed as a ratio in which the number of events is the numerator and the population at risk is the denominator. See also rate. prevalent, adj. Epidemiology. (1) The number of people with a specific condition or attribute at a specified time divided by the total number of people in the population.