The most universal one is the initial waiting period, typically 30 days from policy start, during which no claims are paid at all except for accidental injuries. Beyond that, pre-existing disease waiting periods — often 2 to 4 years depending on the insurer and condition — mean that if you have a known condition like diabetes or hypertension at the time of purchase, related claims won't be honoured until that specific waiting period passes.
There's also a separate category for specific illnesses (things like cataracts, hernias, or certain joint replacements) that often carry their own waiting period, commonly around 1-2 years, distinct from the general pre-existing disease clause.
The practical implication: health insurance isn't something to buy right before you expect to need it — the earlier you buy a policy and let these waiting periods run out in the background, the more genuinely useful the cover becomes when you actually need it.