The animorphs were effected in different ways, and I believe Jake didn't really show symptoms until he was told it was contagious. This makes me think it was at least partially psychological. like maybe Andalites could catch it, so Ax said it was contagious, but it doesn't actually effect Humans as much as he thought it would. This explains why Cassie doesn't get it (besides plot armor, because it makes for a short book if everyone catches it bad).
So if anything, it might have effected some school kids mildly, and maybe one or two had to skip a day.
I got the feeling that the gland is like the Human appendix. Useful in some ways, but not a vital organ, and if/when something goes wrong it needs removing.