Wow, well okay researching this question lead me this which explains how/why Bart turns off your immune system. I never have heard this before.
After entering the body (by whatever means), bartonella infects specialized white blood cells called CD 34+. These blood cells are precursors for cells that line blood vessels and other tissues (endothelial cells). The microbe enters the cell and creates a cyst around itself (called a vacuole) to gain protection. It also turns off the ability of the cell to self-destruct. Chemical messengers stimulated by bartonella microbes cause more CD 34+ cells to congregate. These messengers simultaneously suppress other key parts of the immune response. CD 34+ migrate throughout the body and replace damaged endothelial cells. This places bartonella exactly where it wants to be. Once established inside blood vessels, bartonella starts scavenging red blood cells as a nutrient source.
Bartonella gains protection from the immune system by invading cells lining blood vessels. Symptoms are mostly related to damage caused to small blood vessels (small vessel disease). Types of symptoms are related to the organs involved. Typical organs that can be affected include the liver, spleen, bone marrow, eyes, skin, and the entire vascular system including the heart.