Are Our Physical Ailments Caused by Spiritual Warfare?

Questions about whether our physical ailments are caused by spiritual warfare, how much agency demons have in light of the fact that God sometimes sends them to do things, and how they can be both evil and under God’s command.  

• What do you think about my pastor and his wife attributing any physical ailments they have to spiritual warfare?
• In light of the fact that God is sometimes said to have sent an evil spirit to do something, how much agency do demons have? How can they be both evil and under God’s command?