This is the 11905th most frequent Russian word.
заразностью
The quality of being contagious or infectious.
In this sentence, 'заразностью' refers to the characteristic of the disease that allows it to spread from one person to another, used as an abstract noun.
Заразностью этой болезни можно объяснить столь быстрое её распространение.
The contagiousness of this disease can explain its rapid spread.
Here, 'заразностью' is metaphorically used to describe the laughter, emphasizing its ability to quickly influence the emotional state of others.
Он поражён заразностью её смеxa, который заставляет смеяться всех вокруг.
He is struck by the contagiousness of her laughter, which makes everyone around laugh.
In this context, 'заразностью' is applied to describe the leader's charisma, indicating its influence and ability to garner support.