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 the clearest of empirical falsifications of the the claim of viral contagion and of the pseudoscience of Darwinian virology.
Refer to these documents on Masha and Dasha Krivoshlyopova, Siamese twins born in 1950 and extensively studied by Russian health authorities. One twin would become ill, have fever, pneumonia, or measles and the other would not have any illness, or symptoms.
This is an empirical falsification of contagion in general, and viral contagion in particular.
Their story is also covered in a 2017 article in a UK newspaper:
They had their own hearts and lungs but shared a blood supply. They also had separate nervous systems meaning one could become ill while the other was fine.
One was right handed and short sighted while her sister became a smoker, developed a taste for alcohol, had higher blood pressure, enjoyed clear vision and was left handed.
Only Dasha drank, Masha was unable to because of a gag reflex. Because they shared the same blood supply, both would become inebriated.
Despite the other conjoined twin becoming drunk when one of them consumed alcohol, due to sharing the same blood-supply, when one of them became ill with the cold and measles, the other did not become ill.