Prevention is better than a cure! If cacti and succulents are grown following the advice on these pages, they will be healthy free of pests and diseases ! But sometimes they can occur through poor care – root rot has been mentioned previously (in this article), but probably you have bought or been given a plant that had pests hidden away on it and now they have spread. Always look closely at an unhappy plant – one that is shrinking, lost its healthy sheen or even one that is not growing. Prompt action should save the plant.
Probably the commonest pest is mealy bugs, they appear as fluffy white spots and relish cactus sap! An appropriate spray can be purchased easily from a garden centre or via the internet and all will be well soon.