CS Lewis' Elwin Ransom and SM Stirling's Tiphaine D'Ath physically fight possessed individuals. Ransom, expecting superhuman strength in his opponent, finds to his relief that they are physically matched whereas Stirling's possessed are supernaturally resilient. Poul and Karen Anderson's Dahut dies and returns but, before that, had acted as if possessed.
What happens in real life cases of "possession"? Psychological disturbances, certainly. Is there also literal possession of the organism by another intelligence?