Why would somebody develop allergies after leaving the Military?

I know of many people that have developed allergies since their time in service.

As you get older you can develop allergies to things you previously were not (or only had a minor reaction to). I had no allergies when I was younger. Now at 52 I have reactions at certain times of the year to different things. I don't think it is service related as your questions seems to indicate. Because you left the service and then developed allergies does not create a causation. I left the military and gained weight. Does leaving the military cause the extra weight? My eyesight became worse after I left the service too. Correlation without causation I think.

It's a number of different factors. If you grow up in a certain area you get used to certain things in your area. Then you join and all of a sudden you are in a different environment with hundreds of other people bringing with them whatever they have. In boot camp, that initial illness you get at the beginning is called "the crud", but it doesn't stop there; it can drive your body insane how much you are being exposed to.

The second big factor is that your body changes over time. It's not so much that you develop thing after you leave the military as much as it is that four years (or more) have passed.