Vaccine against polio virus is an example of active immunity.

Explanation

### Step-by-Step Solution:

1. **Definition of Active Immunity**: Active immunity refers to the immunity that is developed when the body produces its own antibodies in response to an antigen. This can occur naturally through infection or artificially through vaccination.

2. **Understanding Vaccination**: A vaccine, such as the one for polio, contains weakened or inactivated parts of the virus (antigens). When administered, it stimulates the immune system to recognize the virus.

3. **Role of Antigens**: The polio vaccine contains live antigens, which are components of the polio virus. When the vaccine is introduced into the body, these antigens are recognized as foreign invaders.

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