Lessons from outbreaks of H1N1 influenza.

A new H1N1 triple-reassortant "swine" influenza virus was recently described in individuals from the United States and Mexico who presented with respiratory symptoms, and the same virus was subsequently confirmed in patients from several countries around the world. The circumstances surrounding the emergence of this pathogen, and the factors that facilitated the initial cross-species transmission, are still incompletely understood. It became apparent in the early days of the outbreak that the virus can be directly transmitted between humans. Pathogens that originate in animal reservoirs and subsequently acquire the potential for human-to-human transmission have caused outbreaks throughout human history. Although each outbreak is marked by its own particularities, it is important to remember the teachings that emerge from previous epidemics and pandemics. Integrating the important lessons of the past will provide the best opportunity to understand host-pathogen interaction and the most powerful approach to implementing effective prophylactic and therapeutic measures.