Would appear that Tom Abrahams so enjoyed writing the Traveler Series as Unprepared, Adrift, and Grounded are stories of this world-altering plague from the very beginning, a prequel. Read them first!
Plot. The Scourge begins even before it has a name, even includes brief flashbacks to civilizations from the year 1000, and before. But, the thrust of The Scourge is the modern-day pandemic, originating in refugee camps. Just as COVID19, modern air travel spreads the disease in days. The characters begin development with innocent trips to the market for beer and subs, a travel jam, innocent every day encounters that escalate to the panic and selfish destruction that is in humankind. It is a period of time from day zero to about 6 months into the pandemic. If you read The Traveler Series first and wonder how some characters got to be where they are, these tales will give you the backstory.
Read them in order to get the most out of them, then on to The Travelers Series for a continued apocalyptic adventure.