Does contracting an illness after imagining the Prophet's biography while in a state of ritual impurity indicate demonic possession or sorcery, and what is the cure?

There is nothing more beneficial in treating obsessive whispers (waswasah) than the Hadith of the Prophet, peace and blessings be upon him: "Satan comes to one of you and says: 'Who created such and such?' until he says to him: 'Who created your Lord?' When he reaches that point, let him seek refuge in Allah and stop." So, the cure for waswasah is to turn away from the devilish thought and seek refuge in Allah the Almighty. As for persistent thoughts caused by a misconception, their treatment is to learn what dispels and invalidates them, along with seeking refuge in Allah the Almighty and asking Him for steadfastness in faith and certainty.