His saying: “There is no contagion”, he means by it that nothing transmits [what it has] to anything else such that harm comes from its direction. Rather, it is through the decree of Allāh, the Majestic and Mighty, and from His prior ordainment. For this reason he said: “So who passed it to the first one?”

He is saying that the first camel to get scabies among the camels, there was no camel with scabies before it which could pass the disease to it through contagion. Rather, when the scabies first appeared in the first camel, it was by the ordainment and decree of Allāh. So it was likewise with the disease that appeared in all of the camels thereafter.