Not all physical or mental illness is caused by demons. The gospels make a clear distinction between illnesses with a demonic base and those without. Nevertheless, some illnesses and distresses do have a demonic foundation.
Satan can control a person’s body but the battle is mostly won in the mind (Philippians 4:8; 2 Corinthians 10:5; "sound mind" - 2 Timothy 1:7).
In some cases demons are said to be the cause of physical distress:
- Deafness and muteness (Matthew 9:32-33; Mark 9:25; Matthew 12:22; Luke 11:14).
- Blindness (Matthew 12:22).
- Convulsions/seizures (Mark 9:20).
- Crippling (Luke 13: 16).
- Violent action (Luke 8:26-29),
- Back problems (Luke 13:11).
- Superhuman strength (Mark 5:4).
In some cases demons are said to be the cause of mental or psychic disorders:
- Torment (Mark 5:5).
- Mental disorder (John 10:20),
- Depression (1 Samuel 16:14-16,23).
- Physical harm to oneself (Mark 5:5).
- Suicide (Mark 9:22).
Satan and his demons also inspire a spirit of fear (Romans 8:15; 2 Timothy 1:7).
This might include a fear of death (Hebrews 2:14-15), fear of man (Proverbs 29:25), worries and anxieties (Philippians 4:6; 1 Peter 5:7), and night terrors (Psalm 91:5).