It's impossible to really say, without testing, whether someone had H1N1 versus some other flu strain (and there are others out there, but not common right now, like an H3N2 strain) versus something entirely different which also gives you flu-like symptoms.

One difference between H1N1 and your typical seasonal flu is more nausea, vomiting, and diarrhea with many H1N1 cases. But then again, there are other bugs out there that can give you these symptoms. Additionally, a certain percentage of H1N1 cases don't develop a fever, which is unusual. So, it's very hard to diagnose H1N1 based solely on symptoms.

And unless you had a laboratory confirmed case of H1N1, no one should assume that they're already immune to H1N1 because they presumably just got over a case of it. In reality, you may not have even been infected with influenza at all.

There was that CBS news report recently where they tallied lab results from state labs. A rather eye opening report, actually. We've been hearing that the CDC's own lab has consistently found almost only H1N1 in the samples it tests. But from April through June, these state labs showed remarkably few H1N1 cases in their samples. Very odd. Without knowing more about the underlying data, that's very conflicting results. There's probably a good explanation, but reports like that really must make life difficult for the health authorities who are trying to keep up some momentum with the vaccine campaign, public awareness about hand washing, etc.

Along the same lines, there was a new study just out yesterday pointing out that old folks are actually at rather high risk of dying BUT out of those actually hospitalized. Few older folks are being hospitalized for H1N1, but the general message seems to be that suddenly old folks are at high risk. Which may make the older generation suddenly flock to flu clinics and demand their H1N1 shots all of a sudden, putting even more strain on flu vaccine supplies. Yeah, must be a headache for public health authorities.