Bodies are made to be able to store mana, some more than others, depending in species and other things. But everyone has a limit, and Mana excess is what happens when one crosses the limit of mana able to be stored on their bodies.
A body can store the external mana for later use, either as an energy source for cells or for spells (in case of Ithakill and animas). This mana is stored in specialized cells, mostly found around the brain, the stomach, and in the arms. But there is a limit to that, and the body cannot stand an excess of mana.
To avoid the damage mana could cause by freely moving around the body, the mana is accumulated in non-specialized cells, causing them to grow and divide faster while trying to deal with all the excess, becoming a benign tumor. This mass can appear everywhere on the body (causing its own separated problems) but tends to appear on muscles, using cells that are specialized on fat storage.
There are a lot of symptoms that can cause this condition, most of them being symptoms caused by the tumors location. For example, a tumor located in a blood vessel, partially or totally blocking it, will cause symptoms related to blood not reaching body parts, but not symptoms related to a skin tumour, where you can see it growing.
As general rule, the skin nearby where the tumour is located will be softly coloured in different colours. In species that have fur or scales covering the skin, this colour change will be harder to see, as it only colours the skin and not the fur or scales.
The condition will pass and heal without any needed treatment, just like a cold, but there are some treatments that can help it leave faster.
While this condition also affects flora and fauna, barely is know of the effects and symptoms in them, other than the ones shared with Ithakill.
It is known that when animals are affected by this condition, they will isolate themselves from others and will appear to be sick, moving way less and sometimes will not eat or drink.
Flora are the group less known about. It is believed that the condition affect them less than others, as most flora can use magic as energy more effectively than other species. It is thought that this condition might be a cause of withering.
I love how you've tied such a common theme as mana to condition. Never have I thought of having too much mana (I always have too little. xD), so this is certainly a great way to put a twist on it. Keep up the good work! :D
I'm glad you liked it! I've had this condition thought for a long time, about time to give it an article!
Love to code, but this one is driving me crazy!
My world Shattered won as the "Most ground-breaking premise new world"!