Advise about a scientific scenario about vaccines!! What would you suggest?

A company that produces vaccines has received a number of enquiries from local residents about their factory. Residents want to know what the factory does and why its product is important. You have been asked to write a short piece of text for a pamphlet that the company will distribute to local residents. I should describe what a vaccine is, how they are produced and why they must be produced in a clean factory. I should also explain how a vaccine works in the context of what we know about the immune system.