The major cause of a labor shortage and subsequent demand for higher wages by serfs during the Middle Ages was the Black Death, also known as the bubonic plague. The Black Death was a devastating pandemic that swept through Europe in the 14th century, resulting in the death of an estimated 25-50% of the population. With a significant decrease in the labor force, there was a shortage of workers, and serfs realized they were in higher demand. This led to demands for better working conditions and higher wages as feudal lords struggled to find enough laborers to work the land.
