The previous answer is right, but to simplify, passive is when you're being protected by someone else's immune system, active is when your immune system does the protecting. How can someone else's immune system protect you? If a baby is getting milk from his/her mother, the milk can have antibodies that can protect the baby is one typical example. The problem with a passive immunity is that it only lasts a very short time (once a baby stops getting milk from his/her mother, that goes away). An active immunity, however, stays with you for years, and in many cases for life (it's how vaccines work).
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