http://news.ucdavis.edu/search/news_det ... o?id=11254
The bacteria initially trigger a strong immune response in an infected animal, but findings from this study indicate that the bacteria soon cause structural abnormalities in “germinal centers” — sites in lymph nodes and other lymph tissues that are key to producing a long-term protective immune response.
For months after infection, those germinal centers fail to produce the specific cells — memory B cells and antibody-producing plasma cells — that are crucial for producing lasting immunity. In effect, the bacteria prevent the animal’s immune system from forming a “memory” of the invading bacteria and launching a protective immune response against future infections.