One of my cadets got rejected as he had asthma as a child but because he was still prescribed inhalers until less than 5 years ago he was rejected and told to reapply when the five years was up. The reason is a clear one, if he was out on operations and his medication ran out or was lost and he has an asthma attack, he is now a liability. The same can be said for hayfever, if you run out of medication in a foreign climate full of pollen or things that cause it to flare up, how can you operate effectively? How can they get the medication to you quickly? Glasses are completely different, your life is not threatened if you lose them whilst out in theatre. It's harsh as yes, serving personnel can contract conditions that require medication but by not recruiting those with the conditions you minimise the risk.