What is All Might's secret injury?

Five years before the main story, All Might fought Toxic Chainsaw — but his defining wound came from a battle with All For One. That fight left him with a destroyed stomach and damaged respiratory system, costing him half his organs. The cheerful giant most heroes see is actually a gaunt man held together by willpower. His muscular hero form can only be maintained for a few hours each day, and that window shrinks throughout the series.

How does All Might hide his weakness?

Discipline and performance. All Might schedules public appearances around his limited "powered-up" time. He maintains the illusion so completely that almost no one suspects the truth — not even many pros. Only Gran Torino, Recovery Girl, and eventually Deku know the full picture. The mental effort required to maintain this facade, on top of active heroism, is staggering.

Why does All Might hide his condition?

Because the Symbol of Peace cannot appear weak. His value isn't just his power — it's the deterrence effect. Villains think twice when they believe the world's strongest hero could appear at any moment. If that certainty crumbles, the entire equilibrium of hero society crumbles with it. All Might carries the secret as a burden of office.

What happens when All Might finally loses his power?

His United States of Smash against All For One expends the last ember of One For All. He reverts to his emaciated true form — publicly, on live television, in front of millions. Then he points at the cameras and says: "You're next." It is simultaneously a threat to villains and a farewell. The Symbol of Peace era ends, and society must learn to be brave without a safety net.

What does this teach about carrying impossible burdens?

All Might's arc is a meditation on what happens when one person tries to hold up an entire world. The lesson isn't that this is admirable without cost — it nearly destroyed him and left hero society fragile when the crutch was removed. True strength means building others up so your absence doesn't cause collapse. He eventually learns this through Deku.