Upon reading this my instant thought was to what Andrea Junker referenced. Pint a Day Pete is so clueless he isn't familiar with the fallout of the Flu.

Here's my thoughts:
1. Waves of the Flu are inevitable.
2. Soldiers "work" together and often live in close quarters.

Even if it is a normal flu season... If you are a fighting soldier and in the middle of a wave of flu, do you want:
A. 30-70% of your unit sick with the flu in the field with you fighting an enemy.
B. 10-20% of your unit sick with the flu in the field with you fighting an enemy.
Ideally you want zero but the flu vaccine doesn't work that way. It's a no brainer for B with the two choices.

Taking it further, what about the pilots providing ground support?
The drone pilots providing you support?
The air defense teams protecting you while you sleep?
The general headquarters people supporting you?
The list goes on and on.

People make mistakes and sick people make a lot more. Often they are not capable of the job period.