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.