Some times sickness comes from satan and sometimes it comes from our bad habits like (drinking, smoking, overeating, etc.) God gives you instructions in the Bible on what to avoid in your life so you will stay healthy and also instructions on how to heal your sickness when it occurs and bring you back to a healthy state.
God says very plainly that He wants you to be healthy and happy. 3 John 2 Beloved, I wish above all things that thou mayest prosper and be in health, even as thy soul prospereth. And so God has made provisions in His Word to keep you healthy . Sickness sometimes is cured instantly, but most of the time it occurs when we follow God's instructions and our body heals itself naturally.
PSALM 107:20 HE SENT HIS WORD AND HEALED THEM, AND DELIVERED THEM FROM THEIR DESTRUCTIONS. Who forgives all your iniquities, Who heals all your diseases,