Health officials worldwide are anxiously watching every case of bird flu. They are afraid the virus will mutate so it could spread from one person to another.

About half the people who have caught the bird flu have died. There is no vaccine and not enough medicine to fight it, which is why some American Army doctors are studying the deadliest flu pandemic in history.

In 1918 a flu virus killed between 20 and 50 million people worldwide. 

Andrew Jakomas, a survivor, describes it, "We had little caskets for the little babies that stretched for four and five blocks.  Eight high.  Ten high."

Why was this flu so deadly, and where did it originate? That's what Dr. Jeffery Taubenberger with the U.S. Armed Forces Institute of Pathology wants to find out. "We want to derive lessons from what we study about the 1918 virus to help us understand how influenza pandemics might form for the future and what we might be able to do to prevent them."

Scientists set out to reconstruct the 1918 virus in the lab.  They found tissue samples from World War I soldiers killed by the flu. No one had isolated or saved the 1918 virus. But the U.S. Army saved autopsy tissue from the soldiers, and Dr. Taubenberger said scientists pieced together fragments of the virus and now have sequenced all of its genes. "It's like doing a jigsaw puzzle or putting together a mosaic."

Last year, British scientists discovered how the 1918 virus, also bird flu, was able to spread to humans and explode into an epidemic.

Sir John Skehel heads Britain's National Institute for Medical Research.  He said British researchers were able to determine the structure of one of the virus' proteins. "This protein is important because it is involved in sticking the virus to cells.  So, for example, when viruses come from birds into humans, this protein has to change a little, so that now, instead of just binding to bird cells, it can also bind to human cells and infect them."

This has not yet happened with the current strains of bird flu, but health officials are concerned that it could.

And chickens and ducks are not the only potential carriers according to Dr. Taubenberger. "There is an enormous reservoir of viruses out there in the world and we are really only at the tip of the iceberg as to how many viruses there are."

For the past several years, doctors have warned that another flu epidemic is bound to happen.  If they can find out how the 1918 virus mutated, they might learn how to disable the current viruses and prevent a new pandemic.