This paper did, however, comment that avian flu, if "serially passaged" through ferrets could develop airborne transmissability while retaining virulence (ie harm). More on ferrets, later.
Evolutionary theory informs us that pathogens (eg viruses, bacteria) are unlikely to kill the host rapidly if there is a penalty; they would "prefer" (or, evolution will "favour" them) to be longer living, and slower killing, with more chances for reproduction, by spreading to another animal.In fact, from an evolutionary perspective, nether sickness nor death are necessary, though sickness may be useful as symptoms such as coughing and sneezing enhance the rate of infection.
And, "an immense traffic of young soldiers (more than one million of them by September 1917) moving up towards the front; in the hospitals, sick and wounded men and women undergoing treatment up to 23,000 of them at any given time. They depict, in addition, a degree of dangerous overcrowding. A score or so of fit young men in each and every tent, plus the sick and wounded tightly packed together, not only in the tented wards, but laid our in palliasses in the corridors, mess and recreation rooms." See images. Also, see Vera Brittain's book "Testament of Youth" and the film.
Also, note, some think Spanish flu evolved in the US, and then may have been serially passaged through humans in Haskell County, Kansas, an isolated and sparsely populated county in the southwest corner of the state, in January 1918, before intense serial passage in humans in Camp Funston, Kansas, which held an average of over 56,000 troops. The mystery, of course, is why it would start in Haskell County, which at that time lacked intensive animal farming and had a small human population. Although there is no record, I wonder if it could have in fact been brought there from Camp Funston, before returning there (and then on to Europe from Camp Funston.) Very severe outbreaks of respiratory illnesses also occurred in Europe before 1918, earlier in the war.
There is also a misperception that if pathogens cross species they are automatically extremely harmful. They can be extremely harmful to the new species, as occurred with Ebola, but this does not mean they all will be (bird flu being a good example if Palese and Wang are correct - see endnote 1). Nor does such species crossing mean they quick or easy transmission in that new species. (Ebola is highly contagious via direct contact, including sexual contact. Speculation of airborne transmission of Ebola has not been confirmed.)
Endnote 1: Palese and Wang, PNAS 2012 (emphasis added)
"The frequency and certainty with which this staggering fatality rate is reported is troubling when one considers how the numbers are generated; in order for a case to be confirmed by WHO, a person must have an acute, febrile respiratory illness (temperature >38 °C/100.3 °F) with known H5 exposure in the 7 days preceding symptom onset and have molecular confirmation of H5 infection by a WHO-approved laboratory (e.g., virus isolation, PCR, serology) (29). This definition does not allow for asymptomatic infections and essentially requires that a person actively seek medical help at a hospital that is equipped to draw samples and ship them to an approved laboratory. Given that rural populations in developing countries are most commonly affected by H5 viruses, it seems unlikely that even a small fraction of the total number of infected cases has been accounted for under the WHO surveillance system. Also, the fatal cases that have been reported are most likely caused by mega-doses of H5 virus inhaled by the patients, who are living in very close contact with infected poultry. Transmission involving small doses of virus (as observed under regular aerosol transmission conditions) may not result in overt disease. Of the 10 largest studies of which we are
aware (N ≥ 500) (30–39), which document seroevidence in humans for H5 infections, two studies report no seropositivity and the other eight report rates ranging from 0.2% to 5.6%; the studies are mostly conducted in rural areas where H5 infections infections have been previously documented (30–39). Even if only a low percentage of the rural population is asymptomatically/ subclinically infected, the case fatality rate that is offered by the WHO—and that is driving this controversy—is likely orders of magnitude too high. This suggests that more work should be done to determine an accurate case fatality rate for H5 infections, which takes into account mild and asymptomatic cases."
Endnote 2: There is an excellent video about intensive farming and ethics by Marianne Thieme, a human-rights lawyer who is leader of the "Party for the Animals" in the Netherlands.
Endnote 3: While Oxford may or may not share my views about a Spanish flu recurrence being exagerated, he is concerned about the possibility of a modern pandemic - probably more than I am, eg see