So have one type of antibody for current influenza strain, another for measles virus, etc. Travels to lymph node where subsequent events happen. For most diseases, if you survive the first week you'll start to get better. Immunization and Vaccines Goal of immunization is to challenge your body with antigen without causing significant disease. Then inject into body, hope it will still resemble natural pathogen enough to stimulate useful antibodies.
Some vaccines take long time to develop.
Jonas Salk spent 10 years trying to develop useful killed vaccine for polio.