" It was such a subtle thing, that I wasn't even aware it was happening until one day a few months later. That day, I accidentally knocked a cup off of my kitchen table and watched as it smashed to pieces on the floor. A fraction of a second later, it reformed and replaced itself on the table, tea and all. " ~Francis Deranthus, The Marvels of Magic
How magic manifests in individuals is not truly known. Competing hypotheses include:
some higher power, such as a god or fate dictating what the world needs;
proximity to magical influences - this one has the most support among scholars; and
genetic ancestry to magical creatures such as demons, faeries, or even dragons.
Headaches are a very common side effect of the beginnings of the Change. These headaches are symptomatic of the huge architectural changes required in the brain required to process and channel magical energy. Other symptoms of this can include stomach pains and random spasms of arms and legs.
The process is irreversible once it starts; the only way to prevent the humanoid from turning into a mage is to kill them. At the same time, the process can be made more comfortable by removing them from external stresses.
After the initial nausea and delirium, the body begins to store magical energy, rather than be simply affected by it. Random magical effects could affect the world around the new mage, including the anecdote above. After some time - the length differs from mage to mage - the new mage will begin to be able to cast spells consciously. These initial spells will be closely tied with the mage's Affinitys and be very simple, but only practice will be able to prepare them for casting more complex charms. Afterwards, the magic stored would be used up, leading to a profound tiredness only cured by rest. This cycle of effect, fatigue, effect, fatigue will take its toll on the new mage. This is usually when a Master is found to educate the new mage.
As the causes are not truly known, there is no real way to prevent the Change from occurring. This is also due to there being little impetus among mages to try and prevent this; the old proverb of "the more the merrier" is especially true among mages, who feel safer in greater numbers.
The way people react to mages is as varied as there are species of humanoids who exist in the universe. Most are treated with a healthy respect or a wide berth. In a few places - namely Earth - those who practice magic are ostracized from and feared by the rest of society.