。
She seems very healthy.
Literal
She [topic-は] very healthy [seems-そう].
~そうだ on a na-adjective drops the な and attaches そう directly to the stem. 健康 ('health') is one of those Sino-Japanese nouns that doubles as a na-adjective for 'healthy.' The construction conveys 'gives the impression of being healthy' — visual or contextual cue rather than verified fact.