GENERAL BIOLOGY - (Dec/20/2014 )

why do people become seriously ill or die if they go without liquid for a week or so?

They dry up. Biologic processes need water. 

Dear Saurabh vemuri, not sure if you read the replies here at all, but see here too.

Therefore it would be nice if you would notice replies and don't ask twice and finally give meaningful titles not in capital letters. That's helpful and fair for the people who try to help you. Thanks.