Causes. Cancer begins in the body whenever a damaged cell multiplies out of control because it has an error in its DNA sequence. In a healthy person, a damaged cell would normally die to prevent this problem from happening. Or, if it does begin to multiply by some chance, the immune system will quickly come to the rescue to clear it out of the body. Sometimes, people don’t have a healthy immune system though because it has become weaker because of their advanced age. So many doctors associate this condition mostly with senior citizens. That does not mean that it can’t occur in younger folks though.