Novel influenza A (H1N1) infection vs. common influenza-like illness: a prospective study.

Background: On June 11th, 2009 the World Health Organization (WHO) declared the first influenza pandemic of the 21st century. Data regarding the clinical characteristics and course of this viral infectious disease are still being assessed. The aim of this study was to investigate and compare the possible differences in clinical course and outcome between H1N1-positive [H1N1(+)] and negative [H1N1(-)] patients.