In the treatise, Al Razi describes the clinical presentation of smallpox and measles and goes on to indicate that exposure to these specific agents confers lasting immunity (although he does not use this term).
No tratado, Al-Razi describe a presentación clínica da varíola e o sarampelo e pasa a indicar que esta exposición a estes axentes específicos outorga inmunidade duradeira (aínda que non usa este termo).