An agent that is smaller than a virus causes scrapie. It is either a protein called a prion or it is another substance closely associated with the prion protein. Scientists still have a lot to learn about the scrapie agent and exactly how it works.
Tissue valves are made with tissues from porcine (pig) heart valves or bovine (cow) cardiac tissue because they function like human heart valves. Once the tissue is removed from the animal, it is chemically treated to preserve the tissue and prevent immulogic reactions once it is placed in a patient.
Pipe organs are primarily made of wood and metal. Wood used to make parts of the organ which are not visible, such as the action, may be made of plywood or soft woods such as poplar. Visible wooden parts, such as the console, are made from hard, decorative woods, such as mahogany or oak. Wood is also used to make some of the pipes.