What should I do about the many intrusive thoughts that come to me when I see anyone afflicted or ill, as I feel that I will be afflicted with the same thing, and does this constitute a bad assumption about Allah?

To alleviate complaints and worries, one must have a good opinion of Allah, sincerely rely upon Him, and be content with His decree. For Allah is more merciful to His servants than a mother is to her child. Allah Almighty says: "I am as My servant thinks of Me, and I am with him when he remembers Me." Whoever is for Allah as He wills, Allah will be for him more than he wills.

And glad tidings for the afflicted: whoever says, "Praise be to Allah, who has spared me what He has afflicted you with, and preferred me over many of those whom He has created with a marked preference," that affliction will not befall him. Persist in this supplication.

Beware of the whispers of Satan, and seek refuge in Allah from him by saying: "O Allah, Knower of the unseen and the seen, Creator of the heavens and the earth, Lord and Sovereign of everything, I bear witness that there is no god but You. I seek refuge in You from the evil of my soul, and from the evil of Satan and his polytheism."