In Outlander season 4 episode 6, Lord John Grey arrived in Fraser’s Ridge and brought Jamie’s son, William, with him.
In the previous episode of Starz’s Outlander, measles devastated a German settler family, and Jamie found Murtagh, who followed him back to Fraser’s Ridge.
Back at the cabin, John asked Jamie and Murtagh not to mention Arsmuir prison.
As John and William were about to set out for Virginia, Claire realized John was coming down with the measles.