Is it permissible for someone with a contagious illness, such as influenza or a cough, to pray with the congregation in the mosque, thereby transmitting the infection to the worshippers?

An ordinary cough is not an excuse for missing congregational prayer. However, if it is caused by a contagious and dangerous disease, the affected person is excused from attending the mosque; rather, he is prevented from doing so to avert harm from others. Jurists have based this on the prohibition of someone who has eaten garlic or similar pungent foods from entering the mosque, due to the harm caused to people by its odor. Since the harm from a contagious disease is greater, they rely on the Prophet's (P.B.U.H.) saying: "There shall be no harm or reciprocating harm."