Desmond mentions the Influenza Pandemic (1918-1919) in his journal. Lacy is often out nursing patients. The Flu infects William (Desmond's brother -and Desmond is astonished that the Flu would dare to do so!)
Every year, we hear a new round of "Get your Flu shot!" warnings. A lot of this dates back to the Flu Pandemic that erupted at the end of the first world war. The Flu killed more ten times more Americans that the war did. More than 25% of the United States came down with the Flu. The mortality rate was enormous. It was really bad.
The Influenza Pandemic of 1918
This is a easy-to-read brief history of the Pandemic. It's not technical, and has great anecdotes about the pandemic. ("...four women [played] bridge together late into the night. Overnight, three of the women died from influenza [Hoagg]." )
The Great Pandemic: The United States in 1918-1919
This website from the US Dept. of Health and Human Services has a lot of information about the pandemic, including this great poster. Don't worry, it doesn't have a lot of technical babble. There's just more information than the site above, if you're interested in learning more.
At any rate, when Desmond came home from the War, home wasn't a terrifically peaceful or safe place to be. This would have increased his stress level, and aggrevated his PTSD.
Lacy's exhaustion and William's illness would have made the whole family more susceptible to what apparently happened, leading to Desmond's flight from his Boston home, and eventual settlement in the Pacific Northwest.
The short lesson here? Don't be a Newborn. Get your flu shot!