Certified GKH Gardening Expert
It sounds like a virus, though I am not certain as to which one. Some varieties of plants, even within the same family, are more susceptible to fungus and disease than others. Other factors, like pests, can also weaken a plant so that it is affected by a fungus while the plants next to it (who are healthier) are fine. I would treat the plant with a fungicide. That should help it recover.