Weight and Muscle Loss After Pneumonia

This message is for Christian. When getting back to the gym after such an illness and muscle loss, (12lbs), what type of workout would you do to get the muscle and strength back? Before my illness I was using a 3 day full body workout that was working well. I am 45 years old and 5’8’’ and weighed 200lbs before my illness. I feel being around 200lbs is where I feel my best. Also, any supplements that you believe are a staple in your own diet for your well being or immune system? Any suggestion would be much appreciated.
Thank You,