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 rate the number of people in a population who have a disease at a given time; the numerator is the number of existing cases of disease at a specified time and the denominator is the total population.Time may be a point or a defined interval, and is traditionally the former if unspecified.revalence (prevÉlÉns) , adj in epidemiology, all the new and old cases of a disease or occurrence of an event during a particular period. It is expressed as a ratio in which the number of events is the numerator and the population at risk is the denominator.