I’m not sure about H1N1, but typically with other flu, if there is going to be respiratory distress, it sets in at about day 5 of symptoms.
Recently, they discovered an unexpected mitigating factor, that opens the door to a bunch of unknown other ones: arsenic. It somehow inhibits the novel pathogen recognition for some time, and then, when the novel pathogen is recognized, the immune system overreacts.
Typically, this effect would occur with long term, low level exposure to arsenic. City drinking water is carefully monitored for arsenic, and has a very low EPA acceptable level, but well water around the US varies considerably, with some having very high levels. The other big source is arsenic-copper anti-termite wood preservative, which is 90% of the arsenic typically found above ground in most of the US.
Don't use this treated wood around gardens with vegetables you plan to eat. The arsenic can be contained in the vegetables.