Coral and shell cuts are the commonest of mishaps, and become infected if not treated immediately by removing any fragments and dousing with antiseptic. 

Tropical ear is a fungal infection of the ear canal and can be very painful in its advanced stage. Treatment is with ear drops and if you think you might be susceptible, use them anyway after getting wet. 

Animals to avoid tend to be on the small side. Shore divers might encounter the dangerous box jellyfish. Jellyfish found at the reef can cause nausea and raise a painful weal, but they're not life-threatening - wearing a Lycra "stinger suit" or full wetsuit with hood ensures protection. 

Some corals can also give you a nasty sting, but this is more a warning to keep away in future than something to worry about seriously.

In fact, the best protection at the reef is simply to look and not touch, as nothing is actively out to harm you. Brightly patterned, conical cone shells are home to a fish-eating snail armed with a poisonous barb which has caused fatalities in people who've picked them up. Don't: there is no "safe" end to hold them. 
Similarly, the shy, small, blue-ringed octopus has a fatal bite and should never be handled. 
Stonefish are nightmarish creatures, so well camouflaged that they're almost impossible to distinguish from a rock or lump of coral. They spend their days immobile, sucking in anything small and edible that floats past, and protected from reprisals by a series of poisonous spines along their back. If you tread on one, you'll end up in hospital - an excellent argument against reef-walking. 

Of the larger animals, rays are timid, flattened fish with a sharp spine capable of causing deep wounds - don't swim close over sandy floors where they hide. The most commonly encountered sharks are the black-tip and white-tip reef shark varieties, and the bottom-dwelling, aptly named carpet shark, or wobbegong - all of these are small and usually inoffensive unless hassled.

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