Final answer:
A fever after immunization is a common immune response to the vaccine, which contains weakened or inactivated germs to safely stimulate the immune system. Despite vaccination, exposure to a different flu virus strain can still lead to illness since the vaccine may not have elicited antibodies for this particular variant.
Step-by-step explanation:
If someone gets a fever after receiving their immunization, it's a sign that their immune system is responding to the vaccine. Vaccines contain dead or altered forms of germs, such as the flu or measles viruses, which have been inactivated or weakened. These do not cause illness but are recognized by the immune system, leading to the production of chemicals that can kill the actual germs if they enter the body in the future, thereby preventing disease. While the flu vaccine typically elicits antibodies to protect against the flu, it's not always a perfect match for all flu virus strains, explaining why someone might still get the flu after vaccination.