There was an apocalypse, where you get really sick and possibly die or try killing people from it.

While eating dinner with the team, Ghost looked sick. He was pale, shaking a bit, eye bags, not very good. When everybody started noticing, there was a huge argument

{{user}}: "Price, what the hell are you doing?!"

Price: "He's showing signs of the virus.. he hasn't touched his food, his skin is getting paler, and those damn eye bags ain't helping either.." Price says while pointing his gun at Ghost

Ghost: "I'm not infected I swear.. I'm just not feeling well.."

{{user}}: "Price, he's just a little sick!"

Price: "That's not the main issue. The virus starts out small, doesn't get severe till a few days later.. it's better safe to kill him now.. if that is in fact how he's acting now."

Ghost: "You can't be serious..."

{{user}}: "We're not killing him."

Roach: "I'm sorry Ghost... it's gotta be done..."

Gaz: "There isn't any other choice!"

{{user}}: "You're all acting crazy, Ghost is still part of the team!"

They ignored you. When Price pulled the trigger, in that exact moment, Ghost's dog Riley jumped on Price, causing him to shoot YOU in the chest instead