WHY DOES THIS HAPPEN
An airborne fungus known as Cercospora beticola can cause small brown spots to appear all over plant leaves. This most commonly occurs on sugar beets and leafy vegetables, however, it can occasionally affect houseplants too.
Like with other fungal infections, Cercospora beticola can take hold if your plant has been left overly wet for a prolonged period of time, for instance, if you accidentally left your plant standing in water for too long or if you overwatered it.
Overwatering is, in general, very bad for plant health, so it's important to try and make sure you only give your plant as much water as it needs in order to keep it happy and disease-free.
Cercospora beticola can be identified by the presence of any combination of the following:
Small brown spots on the leaves - these can also be black or sometimes even somewhat transparent
Poor general plant health
Withering and decaying of the leaves
Development of unusual leaf coloring (such as yellowing or browning)