"Yes, if you touch the wrong thing you'll get cooties."
If you touch something that's non-sterile, you are contaminated. If you then touch something that's not contaminated (or is currently sterile), it becomes contaminated, and so on. This is why a responsible modification practitioner goes through more than one pair of gloves in a procedure, switching it whenever they have become contaminated and then need to access something which they may not contaminate.
As such, a practitioner has to be constantly aware of what they are touching and what its status is. Any time they touch anything that might be contaminated (for example, a cabinet door), their gloves are then considered contaminated. Any clean object touched after that gets added to the contaminated list (including the client), potentially transferring microbes and other contaminants. Switching gloves halts this process if it is done at the right times.