I have a very strong opinion that’s what’s wrong with people is diet related and very probably the same is true of all the animal kingdom . Processed, chemically preserved food . That and the chemical soup and fog that we live in .
I'm sure that has a lot to do with what goes wrong in us, but things were going wrong and killing us on a regular basis before processed foods and chemical soups. I come from a long line of men who died young from factory related diseases, and of women whose exposure to manufacturing was much less and who lived much longer. I hope my having worked outside factories after the age of 24 buys me more time than the men got. But bodies are bodies, whatever we subject them to, and our ancestors in cleaner habitats died younger than we do, on average.
Our fish are trapped in small closed boxes. If we don't water change, they turn into industrial revolution slums really fast. Think of it - overcrowded, filthy... If we add meds without learning why, if we feed badly, if we do things we haven't figured out are bad yet...
We know water changes help. We know that if you live in a rural region, you are apt to have agricultural industry chemicals and nitrates in your tap water. If you draw water from a large river you will have human medications and micro-plastics. My water source seems wonderful for the fish and registers as very clean, but is less then 5 km from an oil refinery and a pulp mill.
As far as aquatic pathogens go,
@Colin_T lives in a country where they're active all year, I would assume, with the warmer conditions. Where I live, they are at their height now, and the days are already shortening rapidly. In a few weeks, the water will freeze and that will bring changes. Indoor fish aren't as affected, but I wonder how the water is? How much does where we live, with its outdoor conditions, affect our indoor hobby?
These fish could be affected by many things, and unless it's so obvious we can't miss it, all we can do is work to prevent problems as much as we can, by taking good care of the conditions they live in.