Source: Tafsir al-Ahlam al-Kabir (Ibn Sirin)
The sickness of the dead in a dream indicates that they are responsible for their religion (debts/obligations). A headache indicates their shortcoming towards their parents, and hand pain indicates their shortcoming towards their siblings.
Source: Ta'tir al-Anam fi Ta'bir al-Manam (Al-Nabulsi)
Whoever sees a dead person sick needs prayers and charity.
Source: Al-Isharat fi Ilm al-Ibarat (Ibn Shahin)
The sickness of the dead indicates a sin from which they did not repent.