how did jean became a shambler instead of a viral runner in gnb

Jean became a shambler instead of a viral runner because, in the game’s lore, he was infected much earlier and had time to progress past the “fresh” viral stage. The wiki notes that Jean is encountered later as a Shambler in Roscoff, and community explanations say the gap between his earlier appearance and that later scene is long enough for the infection to advance.

Why that happens

• Viral runners are described by players as the “fresh” stage of infection, meaning the infected have only recently turned.

• Shamblers are the more advanced form, so someone infected long enough will no longer look or behave like a viral runner.

• Jean’s later appearance as a Shambler fits the time jump between the events in Paris and Roscoff, which the community says is long enough for that progression.

In plain terms

Jean likely didn’t “skip” the runner stage so much as start it earlier and outlast it. By the time the players meet him again, the infection has had time to mature, so he appears as a regular Shambler rather than a viral runner.

Small lore detail

The wiki also mentions that Jean’s Shambler form has a slightly larger moustache, which is used as a visual hint that a long time passed between his first and final appearances.

TL;DR: Jean is a Shambler because enough in-game time passed for his infection to progress from the viral runner stage into a later, non-viral stage.