Dad's owl found Al in his own Fidelius-protected bedroom within his new house, which was under Severus' Fidelius. Revising the additional layers of security spells sandwiched in between those two Fideliuses, Al peered through the window at the arriving owl until he recognised it, reminding himself to add more security spells for the unknown birds. Or at least the unknown birds' letters. The birds themselves were always welcome.
The owl was fed a fresh handful of treats, and the letter was magically stuck to the wall and sliced apart and stared at until Al made absolutely sure it was sent and written by dad. Only then did he touch it.
Which was probably too soon, because he saw the word 'flu' in the letter and dropped it immediately to cast a cleaning spell on both, the parchment and his hand, just to be sure.
Hi, Dad:
Are you absolutely certain that it is the flu? Make sure she drinks plenty of liquids and don't forget to cast the disinfecting charms on the room to prevent the spread of infection.
I'll meet you for dinner tomorrow at the time previously agreed, but with the reduced participant list. Considering the change of circumstances, we don't have to lock all the doors or cast the silencing spells on the dining room.