1. when someone sneezes you say:
Bless you!
Gesundheit!
2. when someone says 'I'm sick.', you say:
( Gute Besserung!/ Ich wünsche dir / Ihnen gute Besserung)
(That's a pity) Get well soon!
I hope you get well soon.
I hope you feel better soon.
3. When someone says goodbye, you say:
( Pass gut auf dich auf!)
Bye. Look after yourself!
Bye, Mind yourself!
You take care now. Bye!