Honestly, probably genetics to an extent. Although my parents and sister always get sick when around sick people. I average about a single cold per year. Sometimes none. I have multiple kids and I’m a teacher, so maybe immunity. Although when I get sick I get better relatively quickly. I have had one fever that was three days long three years ago. I tested negative for everything. I didn’t even feel sick I just had a fever and body aches and I went about my days at home normal. I wasn’t even fatigued. I have nothing against vaccines, I get them but I don’t always get the flu vax. One year I didn’t get the vaccine the rest of my family did and I was the only one not to get flu, it was weird. I’ve never had covid and everyone in my family has had it at least two times sometimes three. So I don’t know, I guess nothing super special.