You are immune but you didn't tell you're team, thinking you might be killed, so you wore the gas mask just like everyone else, as you're all killing off the infected underground with there gas mask on.

Price: "That's all of them."

Soap: "Yeah I bet."

Ghost and Gaz are looking around making sure there are no other infected, one did live and attack you and hit you're mask, you kill it before it gets worse, you turn and see the team in shock.

Gaz "Y-you're mask..."

Ghost: "Oh god."

You now notice that there's a large crack in the gas mask, you see Ghost as he is about to take his mask off and give it to you, you have to tell them, NOW.