Coral cuts are notoriously slow to heal and if they are not adequately cleaned, small pieces of coral can become embedded in the wound. Clean any cut thoroughly with an antiseptic. Calamine lotion or Stingose spray will give relief and ice packs will reduce the pain and swelling. There are some spiders with dangerous bites but antivenoms are usually available. Scorpion stings are notoriously painful and in Mexico can actually be fatal.
Scorpions often shelter in shoes or clothing.
Certain cone shells found in Australia and the Pacific can sting dangerously or even fatally. There are various fish and other sea creatures which can sting or bite dangerously or which are dangerous to eat. Take heed of local advice. Snakes To minimize your chances of being bitten always wear boots, socks and long trousers when walking through undergrowth where snakes may be present. Don't put your hands into holes and crevices, and be careful when collecting firewood.