Worms

I know I posted a similar question, but now that a neighbor asked again, and I still didnt really have an answer I am going to post again.
WHAT causes worms??
Tapeworms are caused by fleas.
I know kittens are usually born with them, etc..
But say you have a 1 yr old indoor cat, what would cause them all of a sudden?
Does human food? If so why do people feed raw, and cooked meat?
And I do mean the other worms, not tapeworms,

Thanks ahead