Studies have shown that when the body is infected, there is a release of powerful inflammatory cytokines that activate brain immune cells (microglia). When activated, these cells in turn release factors that attract other immune cells (called macrophages) to the brain. The microglia and macrophages release high levels of the excitotoxin glutamate, which damages brain cell connections and will kill brain cells if not stopped.

In one study, researchers found that one of the major inflammatory cytokines that causes sickness behavior was tumor necrosis factor alpha (TNF-alpha), which can reach high levels during infections. The researchers used a special mixture of probiotics called VSL#3, which contains eight different species of probiotic bacteria.

They found that this probiotic mixture could return TNF-alpha levels to normal without reducing the inflammation within the body itself and without correcting gut leakiness.

The probiotic mixture also returned the microglia to a resting state — that is, so that the immune cells did not damage the brain.