MC is an abyss who had never infected anything before. The monsters are scared of him but also attracted to him. MC's infection effect is immortality. The infected would die but would be reborn again... the time interval between death and rebirth was proportional to its strength or size. The big monsters took longer to be reborn than the small ones.

SPOILER...

...

ML felt familiar with MC because he had met MC when he was 8 years old. ML heard MC and went looking for him. MC later helped him get home but ML had a fever after and forgot about their meeting. ML had promised then to come back to MC, so that's why ML insisted on becoming an abyss watcher for Abyss 0 when he was 18.

MC was the "lighthouse" for the monsters. He had the ability to show them the way back home. He and ML went to the bottom of Abyss 1 and MC lit the path. Many of the monsters and abyss left earth and went home. During this time ML died but before he died, he had asked MC to infect him. ML was then reborn many years later and reconnected with MC again.

Shi Yuan is the lighthouse to guide the monster way back home.

I love this fact and the way the story represent that the abyss-es and monsters doesn't belong here, on earth, anymore after the infection and how they've been born by different abyss.

They're all lost fishermen, trying to find they way back home in the endless vast sea of universe.

Also, why do I have a feeling that every abyss, like Shi Yuan, is a road blind? Otherwise how could they're like hopping on one onto another planet in the first place?

Road blind or they're wandered too far from home and ended up getting lost, like the 8 years old little Lu Tinghan in the field of infected Xuejian Flower.