Sorry for the delay in responding. Hearing can actually be related to depression. http://www.aitinstitute.org/ - the website / therapy is marketed towards children with autism as that is the largest group affected, and the most severely. There's an out-of-print book called "Hearing Equals Behaviour" by the researcher who 45+ years ago discovered a correlation between hearing and behaviour. People are 100% going to be depressed if they have a hearing imbalance at 1,000 Hz; Imbalance is if say in one ear the lowest you can hear is 11 Dbs, and the other ear you can hear at 14 Dbs - that shows an imbalance of 3 Dbs. The sound therapy I listed helps let the mind 'release' and equalize those imbalances. If you have an imbalance at 1,000 Hz then medication won't he, cognitive behaviour therapy won't help. I imagine it's possible to not be "locked into" a depressive state, but set into one temporarily with hearing-related issues early on - I'm unsure it would fruitful to investigate further.

It's not likely relevant, really. Sometimes people are depressed after flu or ear infection and attribute it to that.

There's some sense in:

1. If you're stressed and/or avoiding sunlight, you're more likely to be depressed and fall ill.

2. You feel less pleasure while severely sick. Personally, I notice my affect when thinking about everything is dramatically different. I discount it for that reason.

3. Depression apparently involves some feedback loops that can make it a fairly stable equilibrium; so the shift in experience during the sickness might linger (perhaps the isolation and lack of sun while sick lingers).

I don't see any studies showing a strong link. To present my own anecdote, I had a dozen ear infections before my teens and have never experienced depression. This is a very small bit of evidence, though.