Its really hard to say what it is but, on the whole, your plant looks happy. A plant with a virus may not show any outward effects but will be less vigorous. If the leaf continues to turn yellow then fall off, I would say too much water. If the yellow spots start to develop on other leaves, it may be a virus.
A virus wouldn't just show up unless you already have it in your collection. Like people viruses, they are spread by the exchange of bodily fluids - in this case, plant fluids - while repotting, dividing, pruning... Practice good plant hygiene. A lot of times, you will not know a plant has a virus until after it spreads to other plants. Sterilize your clippers between plants, wash your hands a lot, always sterilize used pots, and never reuse potting medium.
