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. The number of people in a population who developed a disease at a specified time, regardless of when it first appeared. This takes into account both old and new cases. It serves as an indicator of how seriously and how long a disease affects a population.