Inspired by Sadarse's coffee mug
I have a mug, which I keep topped up near-constantly on my desk. This is because I am a teacher, and all that talking can take it's toll, leaving me parched.

This mug, visually, is a disgrace. As I am constantly using it, it gets washed up rather infrequently. And for that, you can read 'never'. Brown marks abound all around inside it, and it is generally highly unpleasant-looking. It became somewhat of a running joke among one of the classes that I teach. They would tell me how it was a health hazard, and as a science teacher, I should know better. I ignored these slurs on my good name, and slurped my tea with aplomb.

One day, we were studying microbes, and we did a practical where the students swab around for any nasty bacteria or fungi, grow them up on an agar plate, and therefore see what areas of the lab are cleanest and which should be quarantined immediately. This provided the ideal opportunity to test my mug-based bravado.

And what do you know? Out of the entire set of agar plates, the one from my mug had the least bacterial contamination of anywhere in the lab. Cleaner than any hands, cleaner than any desk top, entirely contamination-free.

Turns out, being dirty is fine.
(Serotonicity was a naughty boy on..., Wed 31 Mar 2010, 18:27, 3 replies)

Aha!
The tannin in the tea must have killed all the creepy crawlies.
(TheWonderfulWombat It came to me in a dream., Wed 31 Mar 2010, 18:43, closed)

not sure about the science behind it
but isn't tea (or something similar) a natural disinfectant? *vaguely remembers seeing / sniffing a small bottle of tea tree oil at one point*
(zenmeban thinks the weather could be better, Thu 1 Apr 2010, 10:16, closed)

Perhaps
the fact that you make tea with boiling water is more to do with it.
( K2k6 has a proper job these days, Thu 1 Apr 2010, 11:36, closed)