Lymphocytic encephalitis

Lymphocytic encephalitis is viral disease that is highly lethal to Cylons. It is an ancient human disease that originally was carried by rodents, primarily rats. A couple hundred years ago, humans developed an immunity to the virus.

The virus is especially virulent in Cylons because there is a bio-electric feedback component to the pathogen. It corrupts how the brain of a Cylon agent manages their immune system. If a Cylon who is infected dies and is resurrected, the disease will follow, infecting the Resurrection Ship and eventually the entire Cylon fleet.

Dr. Cottle believes he can create a simple vaccine that can dramatically reverse to effect on the Cylons. However, they have an antibody in their blood which breaks down the RNA of the vaccine. Because of that, they will need regular close interval injections of the vaccine or they will die.

The strain of the virus that infected a baseship via the Lion's Head beacon is an exact match to one reported over 3,000 years ago, right around the time that the Thirteenth Colony left Kobol (A Measure of Salvation).