Fortunately, what we later learned is that most folks middle aged and older (especially senior citizens) had some degree of immunity due to being exposed to similar enough strains several decades ago. It spread rapidly, though, amongst younger people, such as myself, who had no such immunity to a similar strain. Of course, the positive in that is most younger folks have a strong enough immune system that they were able to handle H1N1 fine other than just being miserable for a few days to a week.
I had H1N1 back in mid October during its raging peak, and while I'm a healthy 27-year-old with a presumably strong immune system, I was pretty miserable for about six days. I probably hadn't had a fever over 100 in nearly 15 years, but this pushed a fever up in the 101+ range for me. What made things worse is a bacterial respiratory infection came in about four days into having the flu and just stalled my progress of fighting the flu so it strung out a few extra days for me. Antibiotics wiped out the bacterial infection, and then my immune system finished off the H1N1.