I have a pretty bloody strong immune system in my opinion. Bugs that take my wife out for 4 or 5 days, sometimes into 2 weeks, generally lay me up for a day if I am unlucky. I can go through most winters with just a bunged up nose and not much else. 
Not so this time! About two weeks after we moved into our current house our whole house was taken out by what I assume was the flu. Both boys, my wife, and me. For me, two days of lying in bed feeling dreadful, a week for my ever suffering partner, and the boys were up and down like yo-yos. What really got me was that the whole thing stopped me being able to concentrate and think for two weeks! I couldn't even look at Steemit in that time. So now I am having to play catchup. This has to have been my worst bout of illness in 15 years, if you don't count recurring labarynthitis.(Uuuurgh!)
I love my immune system. I have been a strong believer in not being too picky when food falls on the floor and the idiom that every man must eat a bushel of dirt before he dies. I feel that by bombarding my system little and often I have managed to passively build a strong defense against most coughs and sneezes and have never suffered from allergies. However this was in the UK. Now I am in Spain I guess I am going to get hit a few more times, as there is a whole new wonderful microcosm of different strains of pathogen just waiting to introduce themselves. I just hope my body can adapt quickly, as being sick sucks.
You could try to boost your immunity in the mean time with supplements or homeopathics.
Yeah, I considered some echinecia and I keep meaning to brew some elderberry cordial and rosehip cordial. Maybe finally get around to that this autumn
Echinacea or thyme infusion should help if taken regularly during the cold season. I add some lemon balm, catnip and valerian to help with the sleep
First step to being healthy is actually taking responsibility for it yourself.