This is super easy:
That's because Mickey already got the virus and now is immune. It was touch and go for a while. Minnie found him passed out and blue. They rushed Mickey to the hospital but there were no more ICUs available, so, he was rushed to a make-shift ICU beds they set up in the abandoned ice hockey arena. He was on a ventilator for 17 days. He then pulled through. But his lungs are terribly scarred and he has trouble still breathing and gets quickly winded. That's why you only see him out for five to ten minutes at a time. Also, his larynx was severely scarred, and that's why he doesn't speak.
Any other questions, honey?