Twice, I've had the actual flu. It is NOT a fun disease. When most people think they have the "flu," what they actually have is a case of food poisoning or a cold. Unless you're flat on your back for three days feeling like you're ready for the grave, it's not influenza.
Perhaps Mark was never as sick as when he had the flu shot. But, that's mild compared to getting the full-blown flu. I guarantee he couldn't say that if he caught the actual disease.
So, I asked my doc what the big deal is with the swine flu. He said that the trouble is that is a rapidly changing sort. It can easily mutate into a much more virulent, 1918-style version. THAT is what the medical community is so concerned about.