The people of Jāhiliyyah considered sneezing to be a disease and treated it as an omen. However, sneezing is in fact a healing and blessing from Allāh for which He is praised.
Ibn al-Qayyim (رحمه الله) said:
From that which the people of Jāhiliyyah used to take as a bad omen and consider to be an evil portent—in the same way they would take good and bad omens through birds —was sneezing...
So when Allāh the Sublime brought Islām and the Messenger (صلى الله عليه وسلم) nullified what [the people of] Jāhiliyyah were upon of misguidance, he prohibited his ummah from believing in evil portents and harbouring omens. And he ordered them to replace their supplication for that which is disliked against the sneezer with supplication for mercy upon him...
Also, the sneezer was commanded to praise Allāh when sneezing because the people of Jāhiliyyah used to believe it was a disease.
One of them would dislike to sneeze and would love that it had not emanated from him due to what it contains of misfortune [in their belief]. And the one sneezing would try to restrain himself from sneezing and expend effort to withhold from it due to the evil belief of the ignorant people among them with respect to it.
Read the full article which contains useful information on the difference between the view of disease and its symptoms (such as fever and sneezing) in the Prophetic Sunnah and what is with the people of disbelief in their crass pseudosciences:
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