From time to time I have helped with the Sahana open source disaster management system. Recently someone asked what sort of symbol should be used to indicate where general humanitarian assistance is being provided, rather than just first aid. I suggested the "red crystal".
A red cross on a white background, which many people think indicates first aid, is actually an international sign for general humanitarian assistance. Use of the red cross and red crescent are reserved under the Geneva Conventions for humanitarian aid in war time and in natural disasters. Because a cross and crescent can have religious connotations the "red crystal" was added.
The usual symbol for first aid, is a white cross on a green background. This should be used to indicate first aid, rather than a red cross.
Computer people should be especially careful using these symbols. Sometimes a black and white cross is used as an icon. Just make sure you don't use a red cross, unless it is for the appropriate purpose (misuse is a crime under Australian and International law).