Bites and stings from venomous snakes and spiders can be lethal due to the toxicity of the venom/poison and the effects it has on the nervous/respiratory system. The venom/poison can also cause an allergic reaction in some people.
Approximately 95% of bites and stings occur to the arms or legs of a casualty. If the casualty receives the correct first aid treatment it is unlikely that the casualty will die.
The venom is not in the blood stream, but in the tissue of the bitten limb. The pressure immobilisation technique is designed to slow down the absorption of the venom from the tissue to the lymphatic system.