The logo for World Diabetes Day is a blue circle. It was developed in 2006 for a campaign for the adoption of a UN resolution on diabetes.

Why a circle? The circle symbol is often found in nature, it carries positive energy. In different cultures, it symbolizes life and health, as well as unity.
Why blue? The blue color of the circle symbolizes the color of the sky and the color of the United Nations flag. And the UN itself is a symbol of the unity of peoples and the only organization that appeals to governments around the world to address the problem of diabetes and change the general trends that impede economic development and cause so much suffering and premature death.

There is even an iPhone and Android app called Blue Circle. Users can take a photo in the blue circle and post it on social media with the #WDD hashtag, adding their personal story to the post.