The following similitude helps explain the nature of the contagionist’s error.
Imagine five people four of whom are thirsty and one is not. The first person has a bottle of water, drinks from it, has his thirst quenched and passes the bottle to the second. The second does the same, he passes it to the third, who does the same, and he passes it to the fourth. However, the fourth had had a very salty meal, and despite drinking much water, his thirst is not quenched. He passes the bottle to the fifth and he drinks and he was not actually thirsty to begin with, and thus no quenching of thirst took place in his body, as it did with the first three.
In each case of quenching of thirst (in the first three people), these were fresh creations, they were fresh instances of quenching of thirst. In each case it is a temporal state (عرض) that is freshly created by Allāh through the sum of its causes, at the appointed time and place. The required condition (thirst) and the absence of barriers (salty meal) were fulfilled, in the presence of one of the causes (water). Through this collection of things, quenching of thirst took place, "by Allāh's decree" for the first three people.
So we say Allāh (عز وجل) created the quenching of thirst in each of the three people separately and as for the fourth, Allāh did not create it because He had decreed a barrier to exist (salty meal) and as for the fifth person, he lacked the required condition (thirst) and hence, the occurrence of the effect did not arise in his body, despite the presence of the cause (water) and absence of the the barrier (salty meal), so Allāh had willed for that not to occur.
The Confusion and Error of the Contagionist:
Now, using our similitude, the contagionist effectively says: The first person “transmitted” his quenching of thirst to the other three people, and that this happened “by Allāh’s will”. This is not true, as nobody transmitted their temporal state (عرض) of quenching of thirst to anyone else. It was always a fresh creation, a new instance in each of the three people concerned.
This very same thing happens with disease, there is no difference between quenching of thirst or disease as a temporary state (عرض).
The example with water and quenching thirst just simplifies the matter, which is why it is easy to understand. With disease, the conditions, requirements and causes are generally more complex, but the discussion is exactly the same. If we cannot say that a person transmitted his quenching of thirst to someone else, with or without saying “by Allāh’s will”, then even more so with disease.
In keeping with our above analogy with water being passed around, this is upon the assumption that the sick person emits a microbe or “virus” that is alleged to be disease causing, though this is disputed,[1] but let’s accept it for argument’s sake and give the contagionist his most favourable position.
Just as with the example with water and quenching thirst, you do not say, “So and so transmitted his disease to so and so”. This is what the Messenger (صلى الله عليه وسلم) forbade, that this should be said, because it is a false statement. No one transmits his state or condition (in this case, illness) to anyone else. Rather, each instance is a fresh creation through the sum of its causes, conditions and event-chains by the decree of Allāh. Even if a cause among the sum of causes (the water, or an alleged "virus") is “transmitted” or passed around, if we accept that, then it is still not the case that “disease” was transmitted. It is only ever freshly created in each and every entity through the sum of its causes and conditions.
Further, adding “by Allāh’s will” does not change the reality, meaning if you say, “So and so passed the disease to so and so by Allāh’s will” or “There is contagion, but it is by Allāh’s will” then you are still in error and in opposition to the clear, unequivocal declarations and judgements of the Messenger (صلى الله عليه وسلم). He said, “Nothing transmits (its illness) to anything else”, he said, “There is no contagion” and he declared “contagion” and “transmission” to be affairs of Jāhiliyyah that people will never abandon.
The notions of “contagion” and “transmission” compete and clash with the Qadar of Allāh. You do not say: "We were given rain through the stars (الأنواء) by Allāh's will. And you do not say: "So and so 'transmitted' his illness (الإعداء) to so and so by Allāh's will", or "The illness spread through contagion (العدوى) by Allāh's will."
The correct statement is:
Allāh initiates and creates fresh instances of disease in each and every entity for whom it has been decreed through the sum of its causes, conditions and requirements at the appointed time and place. Within this is the complete singling out of Allāh in His rubūbiyyah, and it is the proof of the Believers against the polytheists and all the stubborn opposers, as stated by al-Ḥakamī (رحمه الله).
Entertaining the notion of contagion is a subtle undermining of Allāh’s decree. It also leads to superstition, harbouring of omens and baseless actions emanating from unwarranted fear.
For this reason, the contagionist, more so than the non-contagionist, is most prone to being manipulated in this arena, while thinking that he is "taking the means", having fallen into misinterpretation in the issue of the means to begin with.
For greater elaboration refer to: Four Affairs of Pre-Islamic Ignorance That People Will Never Abandon: Including Claims of Disease 'Transmission' and 'Contagion'.