Can anyone please tell me why we use "s" after some illnesses like mumps, measles, etc?
Much of it has to do with the etymology of the words. For example, "measles" comes from Middle English/Middle Dutch masel which was the name for a spot characteristic of the disease. Since there are multiple spots, the plural form was chosen. Mump is an archaic word for grimace (facial distortion, often from pain). Probably because of the painful swallowing associated with the disease, patients had multiple grimaces -- hence, the plural form mumps.