This article, a collaborative effort by 12 scientists active in H1N1 research, presents the facts as they are known at this time, covering research 1918- 2008. Martha I. Nelson et al., "Multiple Reassortment Events in the Evolutionary History of H1N1 Influenza Virus Since 1918."
The apparent disappearance of the virus type for 20 years is not explained, but like some others, if in very low and local populations, it may not be recognized.