In my world, there is a disease present in all plants, animals, fish, insects and birds, which doesn't hurt them but is deadly for humans. What can they do with their diet (consisting currently of practically every edible kind of mushrooms and negligible amount of "clean" apples, of which new trees can't be grown) to not become blind and, in time, dead due to vitamin A deficiency? Is it even possible?
closed as too broad by Mołot, Ash, L.Dutch♦, Vincent, sphennings Oct 29 '17 at 22:46
Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. Avoid asking multiple distinct questions at once. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.
Hmm. Why mushrooms and apples.
You don't need a disease. You just need a world that doesn't produce one needed vitamin.
Now, how to get that one vitamin. You used vitamin A. This is usually formed in the body by splitting a molecule of beta carotene. (what gives carrots their orange colour)
To make things interesting, your world doesn't produce carotenes either, but rather an analog that doesn't work for people. So some manual process is required to extract the carotene analog and convert it to something you can use.
The amount of tech you need is up to you.
Example: Grind the fruit of the Analog to pulp, or freeze it to rupture cell membranes, mix with strong alcohol to dissolve the analog, filter to remove the crud, react the analog with an acid or an alkali, neutralize the alkali, and consume. That's at a level of 'hollow gourd' chemistry. Make it more complex if you want a more technical answer.
The advantage of producing carotene is that you only convert as much as you need. Some amount of vitamin A is stored in the liver, but too much can be toxic. Worst case with an overdose of caretene is that it will dissolve into your subcutaneous fat and make you rather orange and looking like Donald Trump. This takes serious amount. Equivalent to eating pounds of carrots a day.
More about beta caratene: https://en.wikipedia.org/wiki/Beta-Carotene