Why do people die of cancer. what makes cancer deadly

Not all people die from cancer about 50% of the people are able to live. 
For example Lung cancer can cause a lung to collapse.
When you have cancer in any place, it would make is weak and unable to do the things that it has to do for your body to operate. 

Cancer is when your cells multiply uncontrollably and can sometimes create tumors. :/
Old cells do not die, and new cells continue to reproduce. 

So far there is no cure for cancer, only experimental treatments. 

Good Luck!