Curious pre-nursing student wondering about patients unable to speak

[echinacea]

Hi everyone, I hope someone will enlighten me about this. When I volunteered in a hospital there were a couple very very sick patients who grunted and slurred really loud and couldn't speak. They were wide-eyed, intense, like they were trying to yell. It was extremely disturbing. One woman was only about 40 yrs old, skin and bones, nearly bald, diapered, moaning/crying/slurred completley un-understandable. When they brought her food tray (I wondered could she have even been able to feed herself?) she pushed it off the table, angry. Another patient was the same. The nurse's aid saw a fly land on his sheet, she took a towel and snapped it to kill it, hitting him right over his legs. He was clearly upset and everytime she went towards him he let out angry sounds. It was so hectic on that unit I never asked what causes this. Neither looked like they had brain trauma injury, I assume this is a disease process. Then yesterday I was lunged at by a homeless man on the street who was similarly trying to speak but all that came was yelled "ungufadabla...etc..." He was looking me right in the eye and, in empathy/pity/feeling helpless/whatever, I gave him a $5 dollar bill - I don't know why I did that, because what could he do with money in that state? He tried to hug me - or fall on me really- and yelled the same giberish over and over waving and swinging his arms. It must be an incredibly frustrating painful existence, there is a concious person there under all of that. What causes this? I can't imagine having to live in that state.