Deborah hit it right on the nose across the board. While someone coming home from a pet store and taking a shower might be a bit out there, you have no way of knowing where the animals at the store have been. I avoid touching any animals they have while I'm there unless it looks good enough for me to seriously consider purchasing and even then when I get home I won't even go into the room my animals are in until I've washed and sanitized my hands and removed any outer layer of clothing (jacket, over shirt, etc). Not some quick little splash under the water, full on medical scrub. When I go to an expo, I carry my own hand sanitizer and any new animals go into their own room until they're clear for 90 days.
Quarantine is not a joke, either, unless you like spending lots of money on vet bills.
You want to prevent mites from over running your collection? Quarantine.
You want to keep RI from wiping out half your snakes? Quarantine.
Parasites? Quarantine.
Insert other communicable illness? Quarantine.
Are some things still going to manage to slip in? Yes, it is, but by practicing good quarantine habits you'll lower the impact on your collection.