Siamese (conjoined) twins wherein one twin becomes ill with an illness yet the other remains free of the illness, despite sharing the same blood supply is an empirical falsification of the the claim of viral contagion.

This—along with the many early 20th century failed experiments reproducing real-life conditions and interactions to "transmit" flu and other illnesses—is among the many evidences invalidating this superstition. This validates and supports the view of those from the Salaf and beyond who understood the statements of the Prophet (صلى الله عليه وسلم) in this subject matter as a negation of contagion in principle.

As for his (صلى الله عليه وسلم) advice to keep sick camels away from healthy ones and avoiding the leper and not advancing to the land of plague, it is from the angle of preventing situations where people are prone to making errors in causation and start to harbour omens after presumption of contagion and start questioning or finding fault with al-Qadar.