I was wondering also what the vet said is causing the infection.

I have 2 cats with tendencies to ear infections. One gets yeast infections the other gets bacterial infections. Naturally, they couldn't get the SAME type, so I have two different ear cleaners. I have to use the ear cleaners twice per week, all the time, even when there is no infection. That just keeps the ear clean and keeps the environment such that which ever type of thing that cat gets won't develop.

One ear wash is Oti Clens. The other is MalAcetic Otic.
Same routine with each wash. Put in a few drops, use a cotton swab to clean out the ear. And remember to wear your rain gear so you don't get soaked with the inevitable head shake. I'm sorry, I don't remember which one is for yeast, which one is for bacteria. I get them both from the vet. It's stuff they always have, so I just pop in when I need some, no need to phone ahead.

So when you ask if it is something you are doing, maybe the vet just needs to direct you on the ear wash prevention? Once the infection is cleared up?

Using it regularly, the swab always comes out clean, making you think you can stop. Don't stop. I find I can skip one wash (doing 2 per week) and just catch up with the next regularly schduled one and go forward. But if I miss two in a row, I am asking for trouble for the cat.

Hope this helps!
Sandra