Note to Brother is a handwritten note on a sheet of notebook paper by a man named Mark that was was left for his unnamed brother to find. Mark describes how he and a few others were waiting for a smuggler to meet them on the outside of the Boston Quarantine Zone until they heard a military patrol approaching. They panicked and hid in the nearby house in an attempt to hide from the soldiers, before they noticed the house contained Cordyceps spores that infected them. Mark bids his farewell to his brother and explains that he hopes the smuggler will find the note to pass onto him. Since the note is unmoved, it is assumed that Mark's brother never read the note.
Note to Brother is found during the second chapter, "The Quarantine Zone", in the section "Beyond the Wall". This artifact can be found just after Joel and Tess' first encounter with three of the infected. Right before dropping down into the open air from up the staircase, go through the large hole in a wall to the left, and straight across the room will be a door. Go through the door and in the upper far left corner will be a table - the artifact is on top of it, alongside a consumable health item.
We were so close. I'm sitting outside the walls
knowing I'll never see the inside of the zone.
While waiting for the smuggler to show up, we
heard a squad of soldiers approaching. In our
panic, we ducked into this building in hopes of
hiding from them. None of us noticed the spores
until it was too late. We're all infected - we have
a few hours, maybe a day at most. I hope the
smuggler is still coming so that I can at least
pass this note to you.
I should've listened to you and come to the zone
with you when I had the chance. Now it's too
Joel: They got infected trying to sneak into the zone.
Tess: Tsh. As much as everyone complains about it. You got all these poor bastards wishing they were living on the inside.