Gather information on your subject area (including the difference between bacterial and viral infections), your target audience (school age children), and types of PSA's (Print, Radio, Television). Decide which form of announcement will work best with your target audience. Weed through your information and try to find more than one source for each piece. Create a concise announcement that hits the main points and portrays them in a memorable format. Try it out with family and friends and ask for pointers on presentation and quality of information.
The passage saying the earth is round is Isaiah 40:22:
He sits enthroned above the circle of the earth, and its people are like grasshoppers. He stretches out the heavens like a canopy, and spreads them out like a tent to live in.