All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
This Hadeeth was narrated by Al-Bukhari and Muslim on the authority of Abu Hurayrah that the Messenger of Allaah, sallallaahu ‘alayhi wa sallam, said: “There is no Adwa (i.e. no contagious disease is conveyed without Allaah’s permission), nor (any bad omen in the month of) Safar, nor Haamah (i.e. no bird is issued form a dead person’s skull as was widely believed in the pre-Islamic era of ignorance)." A Bedouin stood up and said: “What about the camels which, when on the sand (desert) look like deers, but when a mangy camel mixes with them they all get infected with mange?" On that the Messenger of Allaah, sallallaahu ‘alayhi wa sallam, said: “Then who infected (transmitted mange to) the first?”
In attempt to adapt what seems to be contradiction between negation and affirmation of infection, the scholars said: “What is meant by negation of infection is that nothing has the power of infecting by itself, unlike the pre-Islamic belief that diseases had the power of infecting by itself, without attributing that power to Allaah The Almighty. Thus, the Messenger of Allaah, sallallaahu ‘alayhi wa sallam, invalidated that belief and so he shared food with the leper, in order to show to them that it is Allaah Alone (and nothing else) Who causes disease, and it is He Alone Who cures. But, at the same time, he, sallallaahu ‘alayhi wa sallam, forbade them to approach the leper, in order to show them that this is among the means which Allaah has made as a general cause of the effect (the disease)."
As for sharing food with the disbeliever, the answer is that the disbeliever is not impure in person but in heart and belief; Allaah The Almighty Says (what means): {indeed the polytheists are unclean.}[Quran 9:28]
Based on that, there is no blame upon a Muslim to share food with a disbeliever, be he Christian or else, provided that what is eaten should be lawful, and that there should be no religious violations along with eating, like music, unlawful mixing of men and women, or the like. It is narrated that the Prophet, sallallaahu ‘alayhi wa sallam, was invited by a Jew to a meal of barley bread and rancid fat and he, sallallaahu ‘alayhi wa sallam, answered the invitation. But unless there is necessity to share food with them, it is preferable to avoid it lest it may lead to falling in what is unlawful under Sharee’ah.
Allaah Knows best.