My grandfather got flu in January 1915 and was hospitalized for it. "Mild attack" says the note. But there may be more to this than the obvious.
It seems that the strain of flu that hit the world like a sledgehammer in 1918 might have been appearing in a lesser form in 1915. (Link)
At present, no virologist can demonstrate that the ancestor of the causative H1N1 virus was the product of wartime conditions, though recent viral archaeology and reconstruction have suggested that it emerged between 1915 and 1917, i.e. during World War I.
If so, this might have helped Alec in 1918 when he was very weak after 3 years in France and had been gassed. He might have gained some immunity back in 1915.
The war records are quite skimpy really but they offer all sorts of hints. I looked up Lavington Manor, that had been used as the hospital for the Canadians on Salisbury Plain - to discover that not only does it still exist but is the boarding house for a school, Dauntseys, that looks rather special.
More here about the school - looks lovely.