Allah is al-Darr, that is, the One Who can harm anyone He wants in any way He wishes.
He impoverishes some of His servants or causes them to fall prey to sickness according to His wisdom; so, He determines everything, and He alone facilitates the means of harm as a trial from Him whereby He purges the sins or subjects one of His servants to a test in order to raise his status.
“BE SURE WE SHALL TEST YOU WITH SOMETHING OF FEAR AND HUNGER, SOME LOSS IN GOODS OR LIVES OR THE FRUITS (OF YOUR TOIL), but give glad tidings to those who patiently persevere. Who say, when afflicted with calamity: “To Allah, we belong, and to Him is our return” They are those on whom (descend) blessings from their Lord, and Mercy, and they are the ones that receive guidance.”
The Holy Quran (2-155…157)
“If Allah touches you with affliction, there is none to remove it but He…”
The Holy Quran (6:17)
“Say: I do not control for myself any harm, or can I benefit myself in aught except what Allah pleases.”
The Holy Quran (10:49)