Dr Fellowes, a family doctor, knows that influenza is caused by a 'virus' in 1918. This is highly unlikely as it wasn't until 1931 that the first virus was identified in pigs, and 1933 when influenza in humans was isolated.
Shortly after Hugh goes off to war, stock WW1 footage shows advancing British soldiers wearing helmets, but they continued to wearing flat caps until late 1915/early 1916.