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 and incidence are the fundamental measures of disease frequency, but special names have evolved for these measures, depending on their specific use. All of these tend to be referred to as rates, even though, strictly speaking, they often refer to proportions (cumulative incidence or prevalence).