Slingha,
Yes, when we hear the word "virus" red flags go up. The bottom line is, as was stated, it is ubiquitous, and part of life. It does seem to affect some varieties more than others, and cooler, damper weather seems to exacerbate the symptoms. Those varieties tat are m ore affected are probably weaker to begin with (as illustrated above with the weaker immune system in some people).. And, yes, sometimes one branch seems more affected tan others.
But there are 30-40' tress making hundreds of pounds of fruit a year that are clearly affected.
It exists, seems to have uneven affects, and won't be cured in your or my lifetime.
Sounds scary, but in reality is most of the time a minor irritation. Influenza is an irritation for most people, but it is possible, usually quite infrequently, to die from it.