There were a number of flu epidemics during the early 1900's. The most deadly of these epidemics was called the "Spanish Flu". Recent genetic studies have concluded that this flu was closely related to Swine Flu and is believed to have been transmitted back and forth between humans and pigs several times, undergoing mutation each time before its most savage outburst. The following is one description of the Spanish Flu epidemic:

March, 1918 On March 4, 1918, dozens of World War I recruits in Camp Funston, Kansas, come down with a flu. For two or three days, they are miserable and bed-ridden but soon recover and are back on duty. The virulent flu races through the camp infecting almost 500 people, and killing 48 of them - a high but not abnormal mortality rate for flu in 1918. With millions of men mobilizing for the war effort, the epidemic quickly spreads to other army camps and then to cities around America. Public health officials in Philadelphia issue a bulletin about the flu but few people pay much attention. News of the war dominates the headlines and after a few weeks, the flu epidemic abates and most Americans believe the worst is over.

April to August, 1918 Thousands of infected American troops head to exit ports on the east coast, and sail over to the fighting in Europe. When they land in France, the virus spreads across the continent, infecting hundreds of thousands of civilians and belligerents alike. Thousands die and battle plans are put on hold as hundreds of thousands of soldiers fall victim to the flu. The death rate is still considered normal for the time, but the flu does show a disturbing trend - half of those killed are healthy young adults between 20 and 45 years of age. The flu usually has little effect on the healthy. The belligerents squash news of the sickness so that the enemy will not find out their weakness. But Spain, neutral in the war, writes about the powerful flu, which is thereafter called the Spanish Flu. And then, in the latter part of August, 1918, somewhere in western France probably, the virus mutates and becomes highly toxic.

April to August, 1918 The second wave of the virus hits Europe hard, and then feeds on the 1.5 million U.S. troops who have made the trip across the Atlantic. Troop and supply ships spread the virus. The new mutant flu surfaces almost simultaneously in three places. In Brest, France, the chief landing point for American troops, the first cases are reported on August 22. By Sept. 22, 370 American soldiers are dead. Ships leaving Europe bound for other ports bring the new virus with them. In Freetown, Sierra Leone, two-thirds of the native population gets the disease; 3 percent of the entire population is dead by September. And disturbing reports trickle out of Boston, Mass., in early September that thousands are coming down with the flu followed by severe pneumonia. And with thousands of troops massing to go to war, the virus has a perfect breeding environment.

September, 1918 The new virus enters its most devastating phase from September to November. The flu spreads most quickly among soldiers and sailors, who are forced to live in close quarters during the war. By Sept. 23, some 20,000 U.S. soldiers are infected with the new flu. By Sept. 28, 31,000 American sailors are stricken. By early October, almost 2,000 U.S. sailors and 10,000 U.S. soldiers are dead. Authorities, finally recognizing they have an epidemic on their hands, delay then suspend planned troop call-ups to keep the virus from spreading further. But they are too late; the killing has already spread despite official efforts to contain the virus. The United States is not the only country to realize too late that it has an epidemic on its hands. Using fast and modern shipping and rail lines, the virus races to almost every country on earth.

October, 1918 Ships docked in foreign harbors form beachheads for the virus, which quickly spreads into the interior of the major continents. The United States and Europe, with the most modern transportation systems, are overrun quickly. Signs appear in America urging people to avoid contact with the sick and to wear masks. But the warnings do little. Coffins are piled up on city streets to take away the dead. By the end of the epidemic, the virus has killed some 500,000 Americans. Americans, better fed and healthier than most other people on earth, is relatively lucky. The virus does most of its killing in poor and populous countries

October, 1918 No country seemed safe from the virus. Russia, which avoided the first outbreak, is hit when infected Allied troops invade Archangel to support White Russians fighting the communists. The epidemic decimates Russia but does its greatest damage in China and India, populous countries of poorly fed and impoverished people. Accurate numbers are impossible to find, but some estimates of the dead in India reach 10 million. Even distant Australia, which establishes a strict quarantine, is not spared - the flu eventually hits in 1919. And then, as suddenly as it appeared, the Spanish flu disappears - in its savage wake: 30 million dead

The Spanish Flu virus was not isolated and preserved at the time of the outbreak and scientists believed it had been lost, then in 1997 a team from the USA recovered some of its genetic material. The material was extracted from a female victim whose body was buried in permafrost in Alaska and from samples taken in 1918 from two US soldiers who died in the pandemic. The US scientists reconstructed part of the genetic data (a gene sequence) of the virus and compared this with the gene sequences of other strains of influenza virus. However their analysis did not shed light on what triggered the pandemic or why it was so severe.

In 2001, three researchers at the Australian National University's School of Botany and Zoology discovered that one of the 1918 Flu genes was a hybrid that was produced from parts of two other influenza viruses, in a process called "recombination".

Their analysis indicated that this "gene splicing" occurred just before the 1918 pandemic and one of the two progenitors of the 1918 virus was an influenza strain that probably infected pigs. The results suggest that the outbreak was triggered by the recombination.

"The recombination was within the gene that codes for the haemagglutinin protein of the virus. Changes in this protein are known to increase the virulence of the influenza," one researcher said. "Recombination has not been detected before within the genes of the virus. Now the possibility that current strains may recombine needs to be investigated as new recombinant viruses may pose a threat. We may have discovered part of the reason for the extreme virulence of the 1918 Spanish Flu virus."

Later research has come to other conclusions. In 2005 Terrence Tumpey and a team led by Dr. Jeffery Taubenberger of the Armed Forces Institute of Pathology revive the 1918 virus.

One of the more surprising insights from reviving the virus is that the 1918 flu had a different origin from other recent flu pandemics. Not only did they discover the virus' entire genetic code, they brought it back to life in a tightly controlled laboratory at CDC offices in Atlanta. Reviving the virus has brought new insights into pandemics in general.

Most flu epidemics arise when a human flu strain acquires a few genes from a bird flu virus. The blended human-avian strain then sparks an outbreak. This apparently did not occur in 1918. The Taubenberger team concluded that it was an entirely new, avian-like virus that got into humans directly without mixing with a human strain.

They also determined that one reason it was so deadly was probably because it prompted an overly aggressive immune response. The chemicals produced by white blood cells in response to the infection may have caused much of the lung damage. There is some evidence the current avian influenza acts in a similar way.

This may gives valuable new clue as to why certain influenza viruses are very virulent and might lead to new targets of drug design, where you might be able to actually dampen immune response a bit.

Research also has shown that current flu vaccines provide some protection against the virus, the scientists add. They believe this is because the influenza viruses that circulate today are highly mutated forms of the 1918 virus.