I know many doctors who do not shake hands with people fearing that there might be an infectious microbe in the hand of this person. For this reason, they do not shake hands with people and this is a waswasah (whispering)... the [occurrence of the] whisper [is thought by people to be] restricted to some of the worshippers, but it has passed on to those, to other than the worshippers, to the doctors and their likes...

Thus, when it is established that Zayd from among the people he has, by way of example—Allāh forbid—the disease of tuberculosis and drinks from a vessel, then you can keep away from this, but you should have certain knowledge that he is afflicted with the disease of tuberculosis. As for a whisper, maybe he has it, maybe has it, then this is not permissible, because this is having an evil opinion about your Muslim brother and secondly it comprises cutting of ties of brotherhood between you and your Muslim brother.

However, it is not the case that a healthy person, merely by mixing with a person who has a transmissible disease, that this disease will necessarily transmit to this healthy person. Sometimes it may and sometimes it may not. So when a Muslim falls into a presumption with respect to something that Allāh has created for a far-reaching wisdom, and then he exaggerates in this created thing and describes it with what opposes the reality, then he has fallen into error. And this is what the people of the first Jāhiliyyah were upon, and what most of the doctors are upon in our time as I have just mentioned.