In college and, sadly, beyond, I had to drink everything out of a Nalgene. I guess I didn't "have" to, but since it was the only clean drinking container in my house I used it all the time. Then once when hanging out in the climbing gym's cave, my friend said that he'd noticed the cap of his reliable Nalgene was covered in mold on the inside. I checked mine and found the same thing. He urged me to clean it, but I said I was sure that it's the good kind of mold and left it. Perhaps it was psychosomatic, but a couple days later I started feeling ill. Then I began reflecting on all the other times I'd felt queasy, including hangovers, and decided to blame them all on the mold I'd allowed to accumulate on the cap. So, I took the cap off and threw it away. It ruined some of the flexibility I had in the bottle, but I could still drink out of it.