In a study a few years ago, rats infected with a certain parasite not only overcame their aversion to cat urine, but became attracted to it.

In other words, they lost their natural fear of cats. The parasite is Toxoplasma gondii. About a third of the people on earth are thought to be chronically infected with it, usually without symptoms.

But in the past decade or so, scientists have begun to realize that some people – like rats – can develop serious behavioral problems when the parasite infects their brain. Schizophrenia, depression, suicidality, bipolar disorder, obsessive–compulsive disorder, neuroticism, hyperactivity – all have been linked to the bug in studies around the world.

Now, a team of VA and university researchers has raised the question of whether T. gondii might be partly to blame for the mood disorders that some troops experience after deployment. In the June 2015 issue of Military Medicine, the researchers reported the results of a small pilot study involving 70 women Veterans of various ages.

Eight of the women tested positive for T. gondii, which can be detected in the blood. Six of these eight had been deployed abroad during their military career.

All the women completed questionnaires measuring PTSD and mood symptoms. Those women who were infected scored significantly higher – worse, that is – for depression, anger, confusion, and overall mood disturbance.