CWD and Mad Cow are different diseases. However, they are forms of transmissible spongiform encephalopathy. Both are caused from a mutated protein called a prion.
The best site to get all of the correct info is
http://www.cwd-info.org
I agree with Bill that CWD most likely was from wild animals coming in contact with the remains of an infected animal. Wild deer will eat animal bones for the calcium. If the remains from a hunter killed animal was positive it can be transmitted to the live animal even if the remains have been there for years.
Several states that now have CWD have shown that the source of CWD in their state has not been the result of the Captive Cervid Industry. Now that CWD is a concern, most states and the operators of deer farms have done a great job in monitoring this disease. Unfortunately, sources of transmission out side of the industry are not monitored as well as they could be and this is where problems are occurring.
If hunters see an animal which show the clinical signs of CWD, they should contact their state's game agency. We, as hunters, owe it to the resource to be a part of the monitoring process.