Dear Fr. Fortea, can a demon cause a physical illness?
First, we need to clearly understand that physical illnesses nearly always have natural causes. The belief that sickness has its cause in the demonic world is rooted in a pre-scientific, superstitious 
worldview in which myth is substituted for reason. This being said, if demons exist, then the possibility exists that they may sometimes act to cause illness, but this is an extraordinary occurrence – i.e., it is outside the normal laws of nature. Normally only rain, snow, or hail fall from the sky, but meteorites also fall on rare occasions. It is the same case here; sometimes strange events happen.
St. Thérèse of Lisieux includes a very interesting chapter in her autobiography:
The illness which overtook me came, certainly, from the demon; furious because of your entry into the Carmel [i.e., of her sister], he was determined to take revenge on me due to all the damage that our family would cause him in the future, but he almost made me not to suffer; I went on with my schoolwork, and nobody was worried about me. It was towards the end of that year that I was attacked by a continual headache. This lasted until the Easter of 1883 … it was while I was undressing that I had a strange fit of trembling … I wish I could describe this strange illness of mine. I’m fully persuaded now, that it was the work of the devil … I was delirious nearly all the time and talking utter nonsense … often I seemed to be in a dead faint, without making the slightest movement … it seems to me that the demon had been given power over the outward part of me, but couldn’t reach neither my soul, nor my spirit, except by inspiring me very great fears of certain things. (“Story of a Soul”, chapter 3)
In an extraordinary and unusual way, God can allow a demon to cause a physical illness in a human being. In fact, St. Luke expressly mentions such a case: “And there was a woman who had had a spirit of infirmity for eighteen years; she was bent over and could not fully straighten herself” (Luke 13:11). The Gospel text does not explicitly say that this woman was possessed but that a “spirit of infirmity” (i.e., a demon) was the cause of her sickness. To this, we can add the sickness of Job and the death of Sarah’s spouses caused by the demon Asmodeus in the book of Tobit, as well as the torments of such modern saints as St. John Vianney, the Curé of Ars.
Editor’s Note: To learn more about spiritual warfare and demonology, Catholic Spiritual Direction recommends Fr. Fortea’s excellent book, Interview With An Exorcist – An Insider’s Look at the Devil, Demonic Possession, and the Path to Deliverance.
Art: Dragon detail from
