Google is a very unreliable guide when it comes to deciding whether a word is used correctly or not. A far better guide is a corpus.
COCA gives133 citations of 'uninfected', 45 of 'not infected', 35 of 'non-infected' and none of 'non infected'. This suggests that the one you want is 'uninfected', but I know nothing about the term generally used by specialists in that field.