Time to throw my anecdotal two cents worth in on the CAE discussion.
A few years ago a friend of mine had an experience with CAE progressive pneumonia in a young kid only a few months old. When the kid got pneumonia she suspected CAE because of her experience with mature does but had never heard of the progressive pneumonia hitting a young kid. She had her vet send lung tissue and other samples to WSU. Their finding was that the kid did indeed have CAE progressive pneumonia. They, too, were surprised to find it in such a young kid and added that they hadn't been aware that the progressive pneumonia strain was found in the eastem states. This was the first we had heard of different symptoms being associated with different strains.
Now this is all my impression of my friend's version of the phone conversation with the researcher at WSU - definitely second hand - but apparently they were working on the angle that the different types of symptoms from CAE (hard udders, arthritis, progressive pneumonia, encephalitis) are caused by different strains of CAE. It would be easy to draw from that the possibility that some of the variation in severity, from no signs at all to crippling and death, might also be due to the difference in strains.
In this region, back in the days when we all had CAE, the type of symptom and the severity seemed fairly constant within each different herd. Some had no signs, some had arthritis, some had udder congestion, some had a combination of symptoms. One large herd that had brought in animals from all over the country had all of the different forms and the severest symptoms of CAE that I have ever seen. And, as someone mentioned, herds that suffered the most stress, like show herds, had more symptoms. Those same herds, at least in this area, also had more new animals coming into their herds from different sources. The herd I mentioned with the extreme symptoms rarely showed and the animals were pampered. I showed a lot in those days, 9 - 12 shows a year, but my symptoms and severity remained constant (no arthritis, mild udder congestion). After purchasing my foundation stock from a single breeder I brought very few new animals in and as it happens the later additions were all culled, mostly for rock hard udders. Under the same management and stress level as my home-born does, these purchased does still had more extreme CAE symptoms.
Just thought I would throw in the notion that does that are asymptomatic may not be that way because of or entirely because of inherent genetic resistance to CAE but because the strain(s) with which they are infected is(are) less severe. I certainly think it is possible and worth consideration.