You can develope immunities without ever having been sick. Your body does it every day.
The last time I succumbed to something called a flu was some twenty years ago. And my job brought me in to contact with many people. I probably had many a flu but would get no more than some cold symptoms.
I'm sure you build many immunities in a place like NYC. In fact, I'd like to see statistics comparing the rate of reported H1N1 cases in urban versus rural areas.
We take natural preventatives/immune boosters which probably mask many of the effects of any given illness and make recovery time shorter.
I haven’t missed a day of work in 10 or 11 yrs.
(knock on wood)