This is because [this reconciliation between the ḥadīths] negates contagion in principle, completely, as is stated explicitly in the authentic reports.
Imām Aḥmad (along with Imām Mālik) was upon the view of negating contagion [as a matter of superstition] and eating with the sick (leper) with trust upon Allāh.
The twins differing health patterns present a mystery. Why did one become ill with a childhood disease, like measles for example, while the other did not?
Four [affairs] of Jāhiliyyah that the people will not leave: Wailing over the dead; reviling ancestors; tying rain to the rising or setting of stars, and contagion.