A harmless rehearsal
A vaccine carries a recognisable piece of a pathogen — a protein, a weakened version, or instructions to make one. None of it can cause the disease itself.
White blood cells take notes
Cells at the injection site raise the alarm. Lymphocytes that match the shape multiply, make antibodies, and some become memory cells that persist for years.
Why you feel rough for a day
Aching, tiredness and a slight temperature are the immune response, not the illness. They usually fade within 48 hours, and the memory cells stay behind.