What is an underlying condition?

If over 50% of the population are obese (alright that's both overweight and obese), is it an underlying condition? I doubt that every obese person in the US gets a flu vaccine every year.

It seems that being overweight is a risk factor for fatal outcomes with the H1N1 flu. Whether this is due to underlying asthma (or sleep apnea) is hard to say.  But certainly, there are plenty of overweight people without asthma.

CDC press briefing-May 20

"Richard Knox: Yes, thank you.  I'm wondering why, if you have any information or hypotheses, that we would see so many more cases in Wisconsin and Illinois than in, you know, in California and New York, Arizona, and other places where you might expect it to be the hotspots?  Is there some factor there? 

Daniel Jernigan: That's right.  I think what we're seeing now, because we're looking very closely, is probably what happens each year with influenza in certain parts of the country have disease before other parts.  There may be certain factors or events where there are a number of people that are exposed, and so I believe what we're seeing now is a reflection of those factors and not necessarily a geographic or regional difference based on humidity or temperature or whatever.  "

Knox is a reporter and Jernigan is deputy director for influenza at CDC.