Dictionaries define health as the absence of disease, but that's wishful thinking, for diseases typically begin asymptomatically. You don't know you have them. You might feel fine, but you can't say there's no cancer, occlusion, infection etc. lurking beneath the surface. If something pops up, you might describe it, name it, even fix it, but until you see it, you can't know if it's there. So, feeling fine isn't adequate.
It's actually much worse...