Spanish Flu infected 500 million people circa 1918 and killed 5% of the world's population. Suspiciously, it infected 3 far-flung locations simultaneously and has never been investigated as a possible war crime.
This unusual response meant that the most healthy adults, such as soldiers, were the most likely to die from the disease. The first documented cases of Spanish Flu appear to be among Austrian forces in localized field hospitals: Spanish Flu outbreak in early 1917.
The most potent strain of this flu then hit 3 far-flung locations simultaneously, striking France, Sierra Leone, and Boston, Massachusetts in August of 1918. In November 1918 this disease moved from wartime France for the first time into neutral Spain, a country that was not censoring its newspapers of outbreaks of the flu pandemic. News coverage of this flu increased dramatically when Spain's king later became infected, contributing to the eventual name of the disease.
The early outbreaks of Spanish Flu seem to have occurred in military quarters. A less potent strain infected within mere days some 522 U.S. soldiers on a remote Kansas base in March of 1918, for example.
The symptoms of the Spanish Flu plus the locations and timing of its outbreaks have similarities to what a reasoned observer would expect to see from modern biological warfare: soldiers were the most vulnerable target of the Spanish Flu, the early outbreak locations were military camps, and the complications from the disease were both unusual and deadly. One U.S. Army doctor wrote:
These men start with what appears to be an ordinary attack of LaGrippe or Influenza, and when brought to the Hosp. they very rapidly develop the most vicious type of Pneumonia that has ever been seen … and a few hours later you can begin to see the Cyanosis extending from their ears and spreading all over the face, until it is hard to distinguish the colored men from the white. It is only a matter of a few hours then until death comes…. It is horrible. One can stand it to see one, two or twenty men die, but to see these poor devils dropping like flies…. We have been averaging about 100 deaths per day…. Pneumonia means in about all cases death…. We have lost an outrageous number of Nurses and Drs. It takes special trains to carry away the dead.
In 1918, pathologists were intimately familiar with the condition of lungs of victims of bacterial pneumonia at autopsy. But the viral pneumonias caused by the influenza pandemic were so violent that many investigators said the only lungs they had seen that resembled them were from victims of poison gas.
One of the more interesting epidemiologic findings in 1918 was that the later in the second wave someone got sick, the less likely he or she was to die, and the more mild the illness was likely to be.
This was true in terms of how late in the second wave the virus struck a given area, and, more curiously, it was also true within an area. That is, cities struck later tended to suffer less, and individuals in a given city struck later also tended to suffer less. Thus west coast American cities, hit later, had lower death rates than east coast cities, and Australia, which was not hit by the second wave until 1919, had the lowest death rate of any developed country.
Again, more curiously, someone who got sick 4 days into an outbreak in one place was more likely to develop a viral pneumonia that progressed to ARDS than someone who got sick 4 weeks into the outbreak in the same place. They were also more likely to develop a secondary bacterial pneumonia, and to die from it.
If one believes that lower death rates coming later in contagion are due to secondary infections, while higher death rates from the initial infections are due to direct malicious intent (e.g. dispersal of a weaponized virus), then there are at least 5 major points supporting the theory that the Spanish Flu of 1918 was biological warfare:
#1: Timing of the outbreaks,
#2: Early locations of the outbreaks (e.g. remote military camps),
#3: Demographics of those most afflicted (e.g. soldiers),
#4: Symptoms/rapid progression of the disease (e.g. bleeding from the ears), and
#5: Less virulent sub-infections later in each outbreak.
One suspicious flu victim was Anton Dilger, in charge of Germany's WW1 biological warfare sabotage program.
On September 17, 1918, Lt. Col. Philip Doane forcefully voiced his opinion that the epidemic might have been started by Germans put ashore from U-Boats. Said Doane, “It would be quite easy for one of these German agents to turn loose influenza germs in a theater or some other place where large numbers of persons are assembled. The Germans have started epidemics in Europe, and there is no reason why they should be particularly gentle with America.”
Finally, the 1918 Spanish Flu had the unusual property of being able to infect mice, which typical human influenza strains cannot do (the implication being that the 1918 H1N1 flu wasn't natural, but possibly man-made).