When a person gets sick with the disease despite having been vaccinated against it. Breakthrough infections are uncommon but no vaccine is 100% effective.

Additional doses of a vaccine needed periodically to remind the immune system how to fight a disease. For example, the tetanus and diphtheria (Td) vaccine which is recommended for adults every ten years.

A relationship between two things or a situation in which something could occur based on how our bodies work.

Microscopic one-celled organisms found everywhere including on and in our bodies. While some bacteria are helpful, some cause illness. Examples of dangerous but vaccine preventable bacterial diseases include diphtheria, pertussis, tetanus, Haemophilus influenzae, and pneumococcal disease.

Small white blood cells that help the body defend itself against infection. These cells are produced in bone marrow and develop into plasma cells which produce antibodies. Also known as B lymphocytes