Which of the following best describes the fundamental principle of how most modern vaccines protect people?

• A. Giving a person a tiny dose of a fully infectious pathogen is safe and lets their body build immunity
• B. Presenting a harmless form of a pathogen or a piece of it to the immune system stimulates memory-cell formation without causing disease
• C. Infecting people with a related animal pathogen that causes mild illness gives cross-protection against the human disease
• D. Administering ready-made antibodies provides long-term protection by teaching the immune system to remember the pathogen

Answer: (B)