Prions are normal proteins produced by the body that have taken an abnormal conformation. And, are able to catalyse the production of additional proteins with similar abnormal conformations.
Like viruses, they are not considered alive as they lack the cellular machinery to grow and reproduce, and are dependent on living cells to do this.
Since it is a normal protein with an abnormal conformation, they can spontaneously occur. They can also pass from one individual to another through processes such as cannibalism in which tissues from the dead of one species are consumed by the same species. Most notably in humans, they have been passed between generations by ritually eating the brains of dead relatives. In animals, they have been propagated through the rendering process in which dead animal products are cooked producing fats and meat proteins. Then the meat proteins are fed back to the same species as feed supplements.
As discovered with rendering, they can be extremely heat tolerant. Far more heat tolerant than viruses or bacteria. And, they may be able to pass between species from animals (cattle) to humans.