Quick answer
Выздора́вливай!
vyzdorávlivay — a long word for a simple wish
The options
Выздора́вливай!Get well! — informal
Выздора́вливайте!Get well! — polite
Поправля́йся!Get better! — warmer
Не боле́й!Don’t be ill! — casual and very common
Жела́ю здоро́вья!I wish you health!
A long word, worth the effort
Выздора́вливай has five syllables and the stress on -ра́в-. Say it slowly; nobody minds, and the effort itself is part of the wish.
What you hear back
Спаси́бо!Thank you!
Постара́юсь.I’ll try.
Спаси́бо, уже́ лу́чше.Thanks, I’m better already.
Typical mistakes
Выздора́вливай to your boss
Выздора́вливайте
The -те makes it polite.
Будь здоро́в as “get well”
It means “bless you” after a sneeze
Same words, different moment.
Check yourself
Someone sneezes. What do you say?
Будь здоро́в (or бу́дьте здоро́вы) is the “bless you”.