This sort of question is never simple as there is generally more than one cause to the problem. Often a pest or disease will occur on a plant which has already been weakened by other problems within the growing environment. If the growing environment can be improved then the plants health will improve too.
The affected foliage can be removed and destroyed to restrict the spores from spreading.
Nitrogen is a major plant nutrient that is easily leached from the soil, particularly on dry, light soils or in containers. Symptoms include spindly or stunted growth, with initially a yellowing of the leaves before turning purple, red or orange.
Nitrogen levels can be improved with a nitrogen rich fertiliser, but it is preferable to improve the soil by digging in plenty of well rotted organic matter such as leaf mould, or garden compost to improve both the soil fertility and structure in general. Camellias require a humous rich soil and prefer a shaded position they can actually get sunburn! If the growing conditions are less than ideal, then take steps to remedy this. An annual mulch of organic matter around the base of the plant will also help (but dont mound it around the stem of the plant). Finally, you should always water with rain water instead of tap water as the lime contained in tap water will reduce the acidity in the soil over time.