Peasants in Medieval times had tough living conditions and a hard life from the start. They worked relentlessly, and they received little to no food and so lacked sustenance to provide for them. They were also (partly because of the above reasons) very vulnerable to diseases and plagues.
Medieval medicine was also faulty and not available to peasants most of the time.
(If ur looking for a simple answer, just use that they worked hard, had a poor diet, and were vulnerable to diseases).
:)