Evergreen Euonymus

There are many varieties with varigated foliage so if that what you are referring too, its not a disease. The leaves can change patterns as the plant matures but your shrub looks like it could be winter damage (which doesn’t show up until later when the plant begins to receive more intense sunlight), and/or it could also be getting too much water. The yellow on the leaves is normal variegation. But if the green tissue is also turning yellow, I’d guess over-water and maybe lack of Nitrogen, possibly due to the excess water, but may need some fertilizer. Watch out for scale. Euonymous is a scale magnate but we would have to have a closer look for that pest. It is a brown, round shaped pest that can eventually kill the plant.