In The Naked Now, there's a virus spreading among the crew of the Enterprise. (Essentially the same virus as in The Naked Time: It forces characters to reveal their motivations and allows extreme amounts of character development to happen in a single episode.)
Data is shown, repeatedly, to be electronic and mechanical. We see servos in his limbs, electronic control panels in his head, the device that connects his head to his body, and even the effect of pushing his on/off switch. We later see that his skin isn't biological. (In First Contact we see what happens when Data gets actual skin hooked up to his system so he can feel.) (I also think I remember references to him feeling with microsensors, but I can't remember where or when that was said.)
So how can Data not only contract the biological virus that spreads through the rest of the crew and how can it effect his thinking, which is done with a Positronic brain?