If the same individual is on a later date exposed to the actual Bacteria or Virus, and since he has already a past exposure to the infectious material, therefore his immune system is able to mount a strong immune reactions and prevent the illness from happening. Vaccines are made of either dead or weakened bacteria or viruses. The situation is slightly different in cancers. Cancer vaccines introduce cancer antigens to the patients who already suffer from cancer. Cancer vaccines have not been developed to prevent a cancer from happening in healthy individuals, and in contrary they are used to treat an existing cancer or to prevent a cancer that was already treated from reoccurring.
Cancers are much more complicated than infectious illnesses.
We know significantly much less about cancers than we know about bacteria and viruses. Therefore the cancer vaccines today are at their infancy and we hope to be able to have a better understanding of the interaction of immune system and cancers. This will lead the way to development of better therapeutic methods. With our current knowledge and technology, today we have two basic forms of vaccines. Undefined Multiagent Vaccines were the earliest forms of cancer vaccines which were formulated from whole tumor and cancer cells, using either Autologous or Allogeneic tumors.