Propolis is a pure, organic substance used by bees to prevent bacterial infection in the hive. In content, it's a mixture of resins and balsams, waxes and aromatic oils. The propolis story actually begins out of the hive. Plants such as willow, birch and poplar secrete sap which contains chemical compounds essential for the tree's survival. Worker bees collect this, mix it with their saliva and spread it over the inside of the hive.
Bees use propolis to coat the hive entrance.
Every time they brush against it, it is said, they gain immunity to infection. Indeed it's remarkably powerful. When an animal or an insect dies inside the hive, and is too heavy for the workers to drag out, they cover with propolis to keep it from decomposing and causing infection in the hive. It's also used to fasten things together and fill cracks and crevices which the bees cannot get into, both to stop draughts and to exclude pests. Propolis has been found to possess superb bactericidal and fungicidal properties.