I have a pot plant with Swiss cheese kind of foliage that has got some problem possibly some sort of parasite or fungal infection, I have no idea. It's like red-brown rust that brushes off fairly easily but leaves the leaves looking kind of sickly.
What is it and how should I treat that, to stop it reoccurring?
It looks like a rust problem, a fungal infection, fairly advanced by the look of the underside of the leaves. Try spraying thoroughly with a suitable fungicide which treats for rust, covering the underside and topside of the leaves and all the stems. Remove the worst affected leaves if possible, and always remove any fallen parts of the plant as soon as possible. The other possibility is that it has been infected by a mite which has caused a virus, but try the fungicide treatment.
It might also be worth changing the soil in its pot in case of spores present there too - water only when the surface of the compost is dry to the touch, but not shrunken from the sides of the pot, and water thoroughly when you do it, removing any water left in an outer pot or tray 30 minutes later.
Do some of the larger bumps look a little different, smoother, thicker, maybe sort of waxy-looking, and are harder to get off - probably near or on the central mid-vein of the leaf, on top or bottom? If so, you probably have scale, which excretes the sticky stuff (most of which lands on the leaves and floor below), which then gets eaten by various molds and whatever, causing things like the spots you see.
I'd especially look for Ceroplastes Rubens, aka Pink Wax Scale or Red Wax Scale, which looks like little pink waxy domes.
