My father sometimes doesn't use common sense, under the guise of doing whatever it is he wants to do, because he wants to do it. For instance, their dog has a knee injury and my father lets the dog go up and down stairs by herself, and he throws balls for her even though she skids on the hardwood floors to get them (exacerbating her injury). After dinner, he gives the dog their plates to lick, no matter what they've had, or even if the dog has had digestive issues that day. These are just examples. I don't know what to do with my dad when he's like this. What other signs of possible problems should I be on the lookout for?
And what do you want to do about the current issues?
What you've described is indeed a lack of common sense. Or, more accurately, a lack of conscientious nursing of his dog, though I doubt if he sees it like that; and a lack of regard for hygiene.
[Aside: you've hit a nerve with me. My dog couldn't get out of bed this morning. I am feeling very tender about poor old dogs with creaky joints. But as the day's worn on he's loosened up and is currently having a nice snooze on the rug, you'll be glad to know].
Anyway. If the dog finds it too difficult to get up the stairs on her own, she will let you know. She will sit there looking feeble and appealing until you pick her up. And if she can do it by herself, she should - you don't either want her losing mobility in her other joints. If the knee seriously needs to be immobilised their vet may be able to splint the leg in some way; if the vet hasn't done that, it's probably because dogs do have a good deal of common sense and tend not to do something if it hurts.
But neither of these examples is limited to people with failing mental powers. All over the world, you will also find people who, for example, don't wash their hands after using the bathroom or before eating; who allow their animals to sleep on their beds; who feed their companion animals to excess resulting in health problems such as arthritis.
So I doubt that it's the case that your father doesn't *understand* your point of view. It's that he doesn't *agree*. He thinks you're being a fuss-pot. Plus, he's 75 - too young for mental impairment to be highly likely, too old (not to mention, a bloke) to be as punctilious about pathogens as you are.
You can't just let him be?
Take care of you and don't worry about what ifs.
The other stuff, I’d let go. If Dad has a dishwasher, that will sanitize the dishes. If he doesn’t, wash the dishes yourself before you eat off of them. But make sure he’s not giving the dog onions, garlic or other dog-toxic stuff.