When being vaccinated we are given viruses that have been killed or made 'not working' in some other way. Then we get B and T cells that can 'kill' it.
For example, for polio, memory B and T cells produced in response to the vaccine persist only six months, while tetanus disease requires a booster dose every 10 years, and chickenpox last a lifetime.
So I am assuming the B and T memory cells are being lost for polio faster than for tetanus. I am also assuming that the memory cell losing rate is related to the nature of the virus.
What makes some viruses being 'easily forgotten' and others to be 'remembered' for a lifetime?