In Al Qawlul Badee’ Shah Waliyyullah Muhaddith Dehlawi (Rahimahullah) mentions the following amal from his father Shah Abdur Raheem Muhaddith Dehlawi (Rahimahullah) for chicken pox/ Measles:
Take a thread of blue cotton. Recite the entire Surah Rahmaan. Whenever you come to the verse فَبِأَيِّ آلَاءِ رَبِّكُمَا تُكَذِّبَانِ tie a knot and blow on on the thread. Do this till you complete the surah. Now tie the thread around the patient’s neck. InshaAllah he will recover from his illness.