How can pneumonia make a person barely able to speak and walk? could you tell me more about the pathophysiology or mechanism behind? thank you.

Low oxygen?: One possibility is having low oxygen. A disease such as pneumonia fills the alveoli with infectious/inflammatory debris which prevents gas exchange. Maintaining an appropriate oxygen level in the blood becomes more difficult, especially with exertion. A bacterial infection also causes lots of inflammatory cytokines which can also make you feel unwell.