In very deep temperate forest (5+ days out), what dangers can endanger the life(s) of a group of several fit individuals.
I think a list of dangers is potentially useful, but not a "single answer" question so I'm going to make this a CW -- Add your stuff here:
- Hyperthermia/heat stress/heat stroke
- Falls leading to mechanical injury or head trauma
- Getting lost and dying of dehydration/starvation
- Water born illness through drinking tainted water (protozoa, bacteria, etc.)
- Infection of open wounds
- Poisoning from toxic plants (mushrooms, berries etc.)
- Poisoning from chemically tainted water (alkali, heavy metals, industrially tainted, etc.)
- Flooded streams no longer passable, trapping you in an area (or sweep you into rocks/falls) or drowning or losing gear in flash floods or water crossings.
- Animal danger (e.g. bears, snakes, bees, etc.)
- Fire, such as camp fire getting out of hand, lighting overhead branches and starting a forest fire
- Lightning, if you're on a ridge and a storm comes in quickly
- Dead trees falling in high wind - on the group or on the shelter
- Bogs, swamps, quicksand
I think the most likely is actually hypothermia. All it takes is being a little unprepared and getting caught in wet and windy conditions a little above freezing. If you don't deal with that right away, things just deteriorate and then dealing with it properly gets less and less likely until you're unable to deal with anything at all.
I'd say number 2 is getting lost and not being smart about it. Actually nobody dies of getting lost. The most likely cause of death resulting from getting lost is again hypothermia. You said temperate woods, so it sounds like finding water isn't too hard, and you can go much longer without food.
Next is probably physical accident. Stuff happens no matter how careful or prepared you are. Most of the stories you hear of people falling to their death sound like stupidity is the underlying cause with falling only the means. Yes prepared and experienced people doing the right things can still break a ankle or leg, which then prevents them from getting to a planned shelter or something, and that can be life threatening. However, that is actually quite rare.
Most of the other things in the wiki answer are not serious concerns. Yes, they can all happen, but are either exceedingly rare or easily preventable.