I'm much more worried about your own hygiene standards, if you're seriously worried about this.
As others say, you carry a mat and you put it down on the change table. And your main worry isn't drugs or blood, it's smears of baby poo from people who don't clean up! A couple of weeks ago I had a memorably nasty experience in John Lewis, as a mum sat there calmly feeding her baby, having left the change table looking like an outtake from Mud Wrestling 3. Now, if she'd been a HIV infected junkie, I expect I could have reasoned with her, but as it was she totally ignored my passive aggressive 'goodness, this is dirty' comments, and I just got on with it.