Yes, great article!
The short answer:
"As those (infected) deer live and die and drop out of the population, and as new deer are recruited, that immunity goes away (more quickly). It's got to be sustained for herd immunity to build."
In other words, EHD typically doesn't hit northern herds as hard or as frequently, as in the South, thus northern deer just don't develop as much immunity to EHD. Then when there is a bad outbreak (such as last year's in the Ohio Valley areas), it can be more devastating than what we more often seen in the South.
