Your travel mug is probably full of all kinds of moldy crud.

Now you're saying, Oh no, not me. I washed that thing every day. Look, just because you wash it every day does not mean that you truly got it clean. There could still be a patch of mold hiding down in the deep dark recesses and crevices of your Travel mug. 

According to delish.com.  

"In fact, many travel mugs have removable seals to trap in heat or cold — and if they aren't properly cleaned and dried, these seals also trap bacteria," Delish explains. "As you may remember from science class, bacteria eventually turn into mold in moist, dark areas."
 

The main examples that Delish cited were users of Yeti products. My personal favorite is the Bubba Keg, I get 32 ounces of ice water, I don't have to re-fill it all day. (I think you can still find them at Stan's Merry Mart in Wenatchee.)  

But these problems pretty much go across the board on any travel mug. Just remember, paying extra money for a high-quality travel mug does not necessarily protect you from mold.  

So how do you get that nasty thing clean? Marymaids.com says,  

“Washing the mug out with hot water and dish soap on regular basis. Removing the seal from the underside of the lid will help make sure nothing is trapped up there. 

Let the mug and lid dry separately before screwing the top back on and using the vessel again,"  

You might also consider a deep clean of the cup and the lid Allrecipes.com says.  

“A mixture of 1 teaspoon of baking soda and a 1/2 cup of distilled white vinegar in the mug should get the job done. After the bubbles settle down, cap the mug, leaving the drinking spout open but covered with your thumb or finger, and shake it, pour out the bubbly concoction, rinse your mug with warm water and let it dry before using it for your next drink.” 

I am the worst offender when it comes to cleaning my travel mug partly because all I put in it is ice water, so I just rationalize that it should be OK. 

Wash that travel mug. You will be glad you did.