Strange visitors from the planet Herpes Simplex?

Not a question of cosmic proportion, but nonetheless a question that has plagued me for many years.
There was an episode of the Brady Bunch in which Greg, through expert use of a flashlight and slide-whistle(and IIRC some elaborate system of ropes and pullies used to suspend a common beadsheet above the Brady back yard), is able to make complete fools of, not only his entire family, but some bumbling Airforce big-wig, by causing them all to believe they’re seeing UFO’s. Throughout the entire episode, Greg is clearly seen sporting what appears to be a big, white, puss-filled blister on his bottom lip. What is that?!?
Now I certainly wouldn’t expect to hear Bobby quip something like “Mom always says, don’t kiss the crack-whores”, but no one even acknowledges this painfully obvious growth.
To make matters worse, at one point Marsha becomes privy to the whole wacky scheme and Greg offers to let her try it out. As she brings the slide-whistle up to her face, I can’t help but cringe and shout out “don’t put that thing in your mouth…look at his lip for chrise’ sakes!!”
With the myriad of Brady tell-all books out there (none of which I’ve read) I have to imagine that this must have been discussed. Can anyone tell me, once and for all, what the heck that thing is?