Just my

here. While bacteria and viruses living in the juice is pretty much an impossibility there are still some concerns. The outside of the carto, the drip tip if used, and even the pv itself. These little ickys can live outside of the body for some time and survive on non-porus surfaces.
Think of it this way, the mouth has thousands upon thousands of little icky's living in it. I put my mouth on the drip tip and transfer those ickys to the OUTSIDE of the tip. Those ickys are what can be transferred to others. When sharing a PV, even the act of touching can transfer ickys.
Be safe, don't share your PV's or drip tips, and clean your's on a regular basis.
To the OP, if your friend had some bacteria or a virus on his hands then touched your pv, it is now contaminated. You pick it up, transfer the virus to your hands, and hours later rub your eyes, touch your mouth, pick your nose (lol) and now you have the virus.