I thought 'Ah! I'm being safe this Christmas, not gonna eat too much meat or anything that will kick my stomach off into indigestion/acid' etc and realised that this time last year I weighed a healthy 54kg and don't recall getting sick all Christmas (maybe the odd sore throat and cough) ... I've felt healthier since limiting my meat intake and eating more fish, beans, vegetables BUT its dawned on me......
-I lost a good stone(possibly two) cos of emet and stress this year(its been a downward spiral!) And I'm petrified of what's gonna happen if I get sick. Not even just stomach flu but what if its a proper flu? My appetite is shot down when I'm unwell and no matter how hard I try, I can't seem to gain my healthy weight back. It's concerning but its my stomach/anxiety which holds me back...

So my question is, does being underweight lower immunity? I really really wanna gain weight before proper flu season hits (our family usually get a bad or mild version around new year) and I can't lose more weight

Anyone feel the same? Or have low immunity?
I must say, I've been OK since March in terms of illness....but that one made me feel horrific...aching for weeks and headaches and backaches and mouth ulcers - I think it's the reason for the start of my weight loss as I couldn't eat due to 10 mouth ulcers(which supposedly is the body crying out with bad bad immunity)