My impression was more along the lines of: “I don’t think Brandon realizes that the Common Cold is dangerous and people die from it.” Though we can explain no one in the tower dying of the Common Cold on the basis of: good immune systems, being in overall good health (most of the population are soldiers), and the relative youth of the population at least some (the very young and very old) should be getting more severely sick.
People should still be dying though, even if not many do. About 20 per 100000 (CDC) die from cold related issues. Since Rosharans don’t have preexisting protections to viruses, I’d double it to 40 per million. Considering the tower’s population there should be a notable number of severe reactions and a handful of deaths. So the fact that no one has really gotten ill is very weird to me, because that doesn’t track with the common cold.
Also: Babies should be dying. Straight up. Infants don’t have fully functional immune systems, Stormlight or no Stormlight. Specifically, they struggle to fight off viruses. They get immunity from their mother’s, at least at the start. If mom never had the Common Cold than baby has no immunity to it. And infants are more at risk for developing complications and have weaker respiratory systems.
So the Purelake Plague should be killing infants under a month. The fact that it isn’t seems more of a research failure than anything that has been explained in-world.