I'm not a biologist but I have an idea for a cure for cancer and it is very simple and probably has flaws (if it worked it probrably wouldn't be a cure for all) or is not possible but ... I'm still going to ask why because I want to learn why it doesn't work and I want to learn more in general, so you can use all the biology terms you like. The idea is essentially to use a virus and manipulate/change it (this would be mechanized if it were to be mass produced and the type of virus is unknown to me because you want one that is fast growing and goes across only specific parts of the body depending on the cancer). This virus would not have any bad parts (parts that make you sick) to it because when the virus "injects" it's DNA or manipulates the existing for copying it would instead manipulate the DNA in the cell it is going into to have growth limits and other cancer genes that are set to being back to normal and then copy itself. Eventually it would be fought off by the immune system and most of the virus would be gone (along with the cancer) because this virus would last for a long period of time (week(s)).
I appreciate anyone who answers this question because I'm sure it sounds stupid from your perspective but this is the place to learn.
Note: This is not an idea I'm trying to make, obviously, I just couldn't get it off my mind and wanted to see what is wrong and if it is fixable.