A guy I was in school with joined the Paras. He was in 6 years or so and had perfect health, he made corpora and was expecting a promotion to Sergeant. He was doing a Northern Ireland tour (second I recall) and half way through suddenly got asthma, whilst it was relatively mild they got him out of the army on a medical.
Three years later the asthma had cleared up and he joined the TA as an infantryman - medically cleared 100% fit and no relapses. His consultant told him that in some cases high stress levels can trigger asthma, but remove the stress and the asthma goes away.
Thinking about Bluewolfs post, the adrenalin angle makes a lot of sense - my mate always thought it was a psychosomatic condition, sort of a mental excuse not to run around the streets of Londonderry dodging IRA snipers.