Bacterial pneumonia may have killed most people during the 1918 flu pandemic, and antibiotics may be as crucial as flu drugs to fight any new pandemic, U.S. researchers reported on Tuesday.

Samples of lung tissue taken from soldiers who died in the pandemic, the worst of the 20th century, showed evidence of damage both by the flu virus and by pneumonia-causing bacteria.

Such so-called co-infections also cause many influenza-related deaths today.

"In essence, the virus landed the first blow while bacteria delivered the knockout punch," Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, which conducted the study, said in a statement.

Writing in The Journal of Infectious Diseases, Fauci, Dr. Jeffery Taubenberger and colleagues said preparations for future pandemics should include stockpiling antibiotics as well as antiviral drugs and vaccines.

The researchers also reviewed scientific and medical journals for studies on autopsies conducted on influenza victims from the 1918 pandemic, in which between 50 million and 100 million people died around the world over 18 months.

They consistently found evidence of bacterial pneumonia along with flu in the victims, the researchers said.

Most experts agree that another influenza pandemic will come at some time, although no one can predict when and what strain of flu might cause it. Influenza constantly mutates and pandemics usually occur when a completely new strain gains the ability to easily infect humans and then pass from person to person.

The current chief suspect is the H5N1 avian influenza affecting mostly birds in Asia, Europe the Middle East and Africa. It rarely infects people but has killed 243 out of 385 humans infected since 2003.

Many countries are preparing for a pandemic by stockpiling drugs, developing vaccines and making plans to help society function when many people are sick or staying home to avoid infection.