WEDNESDAY, Sept. 28 (HealthDay News) -- The influenza virus that killed 50 million people worldwide in 1918 was circulating in the United States at least four months before the outbreak reached pandemic levels in the fall of that year, researchers say.

Their finding comes from examinations of preserved lung tissue and other samples collected during the autopsies of 68 American soldiers who died of respiratory infections in 1918.

Proteins and genetic material from the 1918 flu virus were found in specimens from 37 of the soldiers, including four who died between May and August, months before the pandemic peaked.

Those four cases are the world's earliest known documented cases of the 1918 flu pandemic, according to the team at the U.S. National Institute of Allergy and Infectious Diseases.

The researchers also found that the tissue damage and clinical disease in the pre-pandemic victims were the same as in cases that occurred during the height of the pandemic. This indicates the virus didn't undergo major changes that could explain the unusually high number of deaths that occurred during the pandemic.

Like the 2009 H1N1 flu pandemic, the flu of 1918 also replicated in both the upper and lower respiratory tract, autopsy materials from the soldiers showed.

The study appeared recently in the Proceedings of the National Academy of Sciences.

More information

The U.S. Department of Health and Human Services has more about the 1918 flu pandemic.