The World Health Organization expressed heightened concern following the presentation of new evidence on clinical aspects of the H1N1 pandemic by experts from heavily affected countries, which document that “primary viral pneumonia is the most common finding in severe cases and a frequent cause of death.”

Experts confirmed that most people infected by the H1N1 pandemic virus do not experience complications and recover within a week. But they also expressed concerns about small subsets of patients who rapidly develop severe progressive pneumonia.

Pregnant women, children younger than 2 years old, and …