Will there be fewer mathematicians due to fear of brain fog?
If a viral infection could destroy your ability to do math research due to brain fog, then maybe you will avoid this career path?
I think anyone afraid of that will avoid the causes of brain fog (to the extent possible), rather than the enormous list of hobbies or careers that you might no longer be able to do if you did get a bad case of it.
Intellectual hobbies and careers are chosen because life without them is boring, more than because of the money — it doesn’t look like Pratchett was put off writing as a result of her father getting early onset Alzheimer’s, for example.
Is there any reason to believe yourself unusually vulnerable to this virus, or unusually likely to develop brain fog if you did get it? There really isn't any career that can't be wrecked if you acquire a major new disability (through infection, accident, whatever).
Any career path that requires strong cognitive skills is susceptible of being damaged due to brain fog, not just mathematics. Will there be fewer people doing all of them?