The short answer
Persistent hiding is usually about how safe a fish feels, not illness. The common reasons are a naturally shy or nocturnal species, stress from tankmates or surroundings, being newly added, or poor water quality. Add cover, keep the tank calm, and — as with any behaviour change — test your water to rule out the environment.
Some hiding is completely normal
Plenty of fish are shy or nocturnal by nature. Loaches, plecos and many catfish stay tucked away by day, and small or peaceful community fish hide when a tank feels exposed. Adding plants, wood, rocks and caves actually makes fish bolder, because they know they have somewhere to retreat. A bare tank tends to leave fish more nervous.
Stress, tankmates and new arrivals
A fish that suddenly hides may be avoiding an aggressive or boisterous tankmate — watch how the others behave when it tries to come out, and see why is my fish chasing other fish?. Newly added fish very often hide for days while they settle; that’s expected, and good acclimation helps. This is such a common question that we cover it separately in why is my new fish hiding?.
When hiding signals a problem
Watch the water first: ammonia or nitrite makes fish withdraw and stop eating, so test and do a water change if needed. Hiding becomes a worry when a normally confident fish disappears and shows other signs — clamped fins, laboured breathing, or refusing food. In that case compare with how do I know if my fish is sick? and, for anything worsening, ask an aquatic vet or an experienced fishkeeping community.