Any sort of insulated thermos has a contamination issue after drinking with it. Because of its being vacuum, the damped status lasts relatively longer than open-type beverage bottles, promoting a faster rate of bacterial growth. Drinking with a tumbler that is contaminated by bacteria and viruses can cause various diseases.
It’s easy to find tumblers that are long left on the desk without any cleaning done after drinking coffee at an office. When a person tries to re-use it, they usually fill it with hot water, shake it, and flush it out. This can be a temporary way of getting rid of the unpleasant smell and some sediments, but does not solve the bacterial issue.
Even though by knowing the fact that using a sponge with kitchen detergent will completely remove all the stain and bacteria inside the tumbler, it’s quite bothering to carry them everytime you want to clean the tumbler.