>>1259196>>1258365>>1259172EMT here. As long as diabetics control their issue, they essentially don't have any issues to worry about that your average person wouldn't worry about. Asthmatics, however, obviously have issues with their lungs. Asthmatics typically have different triggers, and of course no matter the trigger you wouldn't want to have an attack while diving; how the fuck would you administer your inhaler or any epi, if severe enough? The other main thing is that anatomically your lungs (from trachea all the way to alveoli) are susceptible to issues by low-oxygen-content air, which breathing atmospheric air at ground level isn't bad, but when you're underwater partial pressures of all gases in pressurized atmospheric air all will increase since you're underwater; nitrogen in particular, which takes up ~70% of normal air, starts to really fuck with your lungs and body when it's at higher pressure, often further displacing and preventing proper perfusion of oxygen. Lastly roughly every 50ft or so that you descend has the neurological-depressive equivalent of a shot of liquor, which can severely dampen the compensatory mechanisms your body has in place when it goes into asthma attack / asphyxiation.
>tl;dr Respiratory and neurological issues happen often in normal, fully-healthy patients while diving, and being asthmatic will only amply these issues 
