Answer:
Mumps can cause infertility. By giving a mumps vaccine to a boy, we are preventing the full effect of the disease if the boy has mumps in the future because the body is protected against this virus thanks to the vaccine.
Step-by-step explanation:
Mumps can cause infertility in men. If we give a boy the mumps vaccine, we are giving him a modified version of the virus that is harmless, but that will make the body produce the specific antibodies for the disease. If in the future the boy has mumps it will be less harsh since his body has the antibodies to fight mumps and his fertility will not be at risk.