Chicken pox is infectious as hell and can be very dangerous to older kids/young adults/pregnant females (i.e., pneumonia in 1-2% cases, sets one up for shingles later in life, can cause meningitis and death [uncommon, but]).
Bacterial meningitis -- less infections, but if in a dense residential dorm/barracks situation, generally a good idea to make sure everyone gets immunized. Got myself reimmunized when I started grad school in my 30s: remembered that a couple of college kids died of bacterial meningitis about the same time. Cheap insurance.