When I recognize the symptoms of hypothermia, how should I treat it? And what are the dangers if the symptoms are ignored?
When treating mild to medium hypothermia, you should:
Main thing is to stop and get the patient warm. Drink hot fluids, start a fire, etc.
Severe hypothermia should be treated medically if at all possible. It is suggested that the best/only way to warm a person is for two people, one on either side of the person, to get in a sleeping bag and warm the person with their body heat (i.e. gradually). Do not administer hot fluids or attempt to warm in any other way...Do not make the patient move too much, or you will send the body into shock.
As for dangers, if you ignore hypothermia long enough, the person can go into shock, become semi-conscious, unconscious, etc. So it is very serious and should be treated right away.
Read more: Hypothermia Treatment
Admittedly, my first aid training has lapsed - but the last mountain first aid course I took about 5 years ago made a point of matching the speed of onset.
For example - if a person has fallen into an icy stream, they will have very rapidly cooled. You need to get their core temperature up, fast - the best way to do this is to strip them down out of their wet clothes and dry, and then, while still unclothed (this is not the time to be body shy), into a sleeping bag with someone else.
However, if it is a result of long-term exposure, then you can't just rapidly heat them up (no matter how bad it may seem). If they haven't gone too far (teeth are still chattering, they're still shivering) - extra layers and, a hot, sweet drink can help. Keep them moving.
If they are not shivering anymore, they're pretty much heading into hypothermic shock - at that point, get them into a sleeping bag with another warm body. You should strip outer layers, but t-shirts and shorts can be left on.
If you're in a large enough group and you have to stop, send a party for help.