From HomoExcelsior

1. A preparation of a weakened or killed pathogen, such as a bacterium or virus, or of a portion of the pathogen's structure that upon administration stimulates antibody production against the pathogen but is incapable of causing severe infection. 2. A vaccine prepared from the cowpox virus and inoculated against smallpox. 3. Software designed to detect and stop the progress of a computer virus.

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