This is probably an easy question, but I've yet to find an answer.
Recently I felt under the weather more and more often, especially at the time I was waking up. I assumed this was an allergy-related thing so I did an allergy test and found out that I am allergic to dust-mites. This does explain my condition after waking up, as these mites mostly reside in the mattress and thus affect you during sleep. However, it doesn't explain why this allergy hadn't affected me at all until now and why it suddently appeared after 20+ years. So the question is, is this (and more generally other allergies) a congenital situation, or was it developed due to my way of life, (non) cleaning habits etc.?