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:

1. Deafness and muteness (Matthew 9:32-33; Mark 9:25; Matthew 12:22; Luke 11:14).
2. Blindness (Matthew 12:22).
3. Convulsions/seizures (Mark 9:20).
4. Crippling (Luke 13: 16).
5. Violent action (Luke 8:26-29),
6. Back problems (Luke 13:11).
7. Superhuman strength (Mark 5:4).

In some cases demons are said to be the cause of mental or psychic disorders:

1. Torment (Mark 5:5).
2. Mental disorder (John 10:20),
3. Depression (1 Samuel 16:14-16,23).
4. Physical harm to oneself (Mark 5:5).
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).