I had a 6 month period where I had extremely low vitamin d deficiency from pretty much never being outside for long.
I got sick sooooo much and it actually started effecting my nervous system. I just felt out of it, like an epic hangover, for a month or so and I had trouble understanding people and processing what they said when they spoke to me. Memory went out the window, I pretty much had "brain fog".
It was actually really worrying for me at the time because i didn't know what was wrong. After a heap of blood and neurological tests, i just had to get out in the sun and take supplements and I came good within a month or less.