Mr. Burns is Indestructible

I've never had the flu, and I'm curious to know why. I have a few theories: 1) I think not having kids might play a part, as every parent of young kids I know always seems to be catching stuff from them. Edit: maybe the stress of having young kids also plays a part. 2) I don't think it's the food I eat as I eat a pretty poor diet, but I do wonder if it might be the amount of food I eat as it's relatively little (I'm at the low end of the healthy BMI range), and scholar seems to have a few studies relating constant CR to a better immune system. 3) I'm a lifelong public transport user. One the one hand people say this might help build up the immune system but it doesn't quite fit with 1). 4) I avoid touching stuff like public door handles and taps. 5) Obligatory Simpsons explanation [this video]