Does trying to live healthy make sense considering an imminent bird flu outbreak?

[FlatAssembler]

I happen to know for a fact that this is a much studied subject and the results are inconclusive at best.

You realize the same is true for masks, if not even more so, right?

I have not heard of that. Could it be that vegetarians tend to care about hygiene more and thus have been infected with fewer types of germs in their lifetime? This should not impact one's ability to deal with novel viruses, such as COVID-19, as even a previous infection with another coronavirus that attacks humans offers no additional immunity from COVID-19, much less does infection by some other virus.

Please read carefully:

A long-term vegetarian diet, however, seemed to be associated with a lower diversity of T-cell repertoire

Was hygiene mentioned? Or was a diet mentioned?

Hygiene is an obvious other factor here.