Allergies do some strange things. You can have none at all then all of a sudden they hit you. I got allergic to sagebrush pollen that way. I was out bowhunting and watching the pollen blow in the wind. I'd done that for years, but this time it hit me immediately and I've had problems every since. Poison ivy is also that way. You can be immune for years, then all of a sudden it nails you and you'll react to it from then on.
While backpacking many, myself included, will have the hands swell due to the hands staying low most the time and being fairly inert. Doesn't happen while running as you pump your arms or when I work all day on the jobsite swinging/lifting.
A good cure for that is trekking poles. They keep the arms moving and the blood flowing.
