Each is widely used and I'd appreciate any input on why there are these forms and which might be the best English. Thanks.
Do you mean "inflected," perhaps?
Infected as with a virus for example. I am marking a thesis that uses the term non infeced which does not 'seem' right to me. Googling the words in italics I got 6.5 million hits for "uninfected" and 1.25 million hits for "non infected" and "non-infected". Proabbly Google reads the latter 2 as the same. I can not find any information online on any differences between these terms so turned to this site.
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.
Thanks, I'd never heard of that site. Very useful.