ANSWER...

95 % of the contents of the first jug is drinkable water, but the remaining 5% obviously is not.  If you pick up this jug, you know for certain that you will be drinking 5% undrinkable water...

The second jug contains purely drinkable water by probability 95%. This means that if we had 100 such jugs, 95 of them would contain purely drinkable water, and 5 purely undrinkable. If you pick up this jug, you stand a good chance to drink pure healthy water and save your life!