Do not underestimate this subject!
1) Ruin your morale.... All your hi-tech clothes will smell bad within one week so you cannot live with yourself (or others!)
2) poison yourself with bacteria with dirty hands, and become dehydrated from diarrhoea .
3) infect a tiny wound so it becomes life threatening!
4) Your feet can become infected so you can hardly walk, a total disaster in survival terms! (a bit of mylar blanket in your boots can sometimes protect against frostbite)