In Walt Whitman's "Song of Myself", how can Whitman be of "good health" to the reader in line 12?
The "perfect health" referenced in the poem belongs to the speaker in the poem, not directly to the reader. Of course, the speaker, the "myself" who is the voice heard saying the words of the poem, is speaking on behalf of all humanity, so in that sense the good health does belong to all of us.
The "perfect health" extends to include all facets of the human experience. Whitman celebrates the physical beauty and perfection of the human form, saying, "And your very flesh shall be a great poem" and "If any thing is sacred the human body is sacred." He sees the vulnerability, but also the extreme potential, of the human personality - "I am as bad as the worst, but, thank God, I am as good as the best." He values the sexual impulses and the natural wonders of the world around him and how these feelings enhance the human experience.
Now I see the secret of the making of the best persons. It is to grow in the open air and to eat and sleep with the earth.
Those who follow this pattern, in his view, will enjoy "perfect health."