wikipedia

The invention of dotted patterns is believed to have happened in the Middle Ages, but because of a lack of fabric machines and modern medicine, the pattern of irregularly spaced dots made people think of the rashes caused by diseases like leprosy, syphilis, smallpox, bubonic plague, and measles. Hence, the pattern was associated with plague and uncleanliness. The modern version of polka dots is believed to have become widespread during the Industrial Revolution (c. 1760) and the invention of the first sewing machine (in 1790), when perfectly round and evenly spaced dots could be created because of mechanized weaving.

However, starting in middle of the 19th century, the pattern started to spread among the lower classes of society at that time, who were looking for more interesting colors and patterns but could not afford the silks of the upper classes.