I have several chilli plants in my yard but out of these 10 plants only one plant is exhibiting this problem. These plants are almost 4 months old. Unable to find a match for the photos from any source. If it is due to any factor such as water or nutrient or infection, then it would affect all the plants. Could this be something which may later spread to the other plants? Yellowing leaves with banding
To me, it seems fertilizer problem, because of the first photo, the leaves in backgrounds, with very dark veins. Diseases seldom can do such regular works (but diseases which touch roots or trunk). In any case, it is difficult to know exactly what cause is. Competition between nutrients, excess water (which block roots), acidity, and over-fertilizing could mask the real cause. So I would try to check what I did before, to see if it was some water problem (this mean very huge quantities, so that roots had problems to get nutrients), or over fertilizing (e.g. ash). If you are sure none of that could be the real cause, try to add some fertilizer (not too much the first time). If you have doubts: use some fertile soil (which give some nutrients, but also can balance if there were too much of them).
Plants may respond differently (and 4 months are very new plants), and soil is seldom homogeneous (what you planted before is important). The root development could affect also the speed of symptoms. So different plants could looks different.