My dogs have plenty of space to run and roam so they rarely leave the yard other than to go in the house or to a vet visit. I vaccinate yearly for deadly diseases.
For awhile the house next door to me was occupied by a man I'll call Idiot Neighbor. Nice guy, extremely full of shit, only lies when his lips are moving, but very likeable.
One of my dogs is one of those goofy, friendly, life's-a-party kind of dogs. She loves to instigate "fence fights" with IN's dogs. Both dogs meet up at the chain link fence that separates them, then bark at each other, snarling with teeth showing, for a few minutes, then it's a wag of the tail and off to their own business in their own yards.
Idiot Neighbor goes to the pound and gets another dog. Ignoring the instructions to isolate this dog for 10 days, he tosses her out with the rest of the pack. Within a week my dog has a hacking retching cough. Bordetella or as it's commonly known, kennel cough.
I immediately take my dog to the night vet clinic, where they wait until the room clears before I can bring her in. She gets much better in a few days with treatment. IN decides to "let it run its course" and 3 of his dogs get sick, one has pneumonia as a complication.
My dog caught it face to face with his other dog through the fence. His new pound dog never got sick, however she was exposed at the shelter and shed the virus in her new home and gave it to the rest of the pack, one of who gave it to my dog. My other dog, a pound dog herself, never got sick despite sharing a food bowl, probably because she'd had the virus when she was in the pound and had some immunity.
This was indirectly how one of my horses got strangles. One of his horses would always play fight with my TB over the fence. They'd go for hours, nipping and tearing off fly masks. This horse would also steal the hay out of my horse's mouth, and he trashed the chain link fence enough to be able to reach into BitchMare's water barrel and drink out of it. Idiot Neighbor worked as a farrier, and probably never sanitized his shoes. His horse contracted the virus but never got sick, he passed it on to my guy, who did, thankfully a very mild case. Since during the strangles incubation period my horses weren't visited by vets or farriers and we didn't have a hay delivery, this was the only source of exposure. Surprisingly enough BitchMare, who had a severely compromised immune system, didn't catch it.
This is the reason my work shoes don't come in the house and the boots I wear to help at the horse rescue are only worn there. However, one of my cats who loves to dump over my clothes hamper and sleep in it has gotten a cold twice, probably from my work clothes.
We can't live in a bubble and we can't keep our pets in one either, but exposure is everywhere.