How To Avoid Afflictions
Hypothermia
Caused by being "Frozen" for too long (when your warmth meter is completely drained). Don't spend too much time with an empty meter. This can be easy to accidentally ignore when sleeping. Be sure to watch for the warning that you're at risk of hypothermia. Increase your warmth as soon as possible when you see it.
Frostbite
Caused by leaving part of your body exposed (not wearing clothing on that area) or by wearing frozen clothing. Wear a piece of clothing in every category (both inside and outside slots). And immediately remove clothing that becomes frozen. Wet clothing will not cause frostbite.
Burns
Um, don't stand in the campfire.
Sprained Ankles and Wrists
These are most often caused "randomly" while walking around on slopes. When traveling long distances, try to stay on level ground most of the time. Carrying too much raises your risk of sprains. They can also be caused by falling long distances and by predator attacks.
Blood Loss and Infections
Avoid altercations with predators.
Lacerations and Bruising
These afflictions do not need healed. They just take a chunk of your condition away. Avoid confronting predators and falling from too high.
Food Poisoning
Caused by eating food that's degraded to a low condition. The type of food determines at what percentage condition it starts to create the risk of food poisoning. Don't eat very low-quality food if you want to avoid this. But the risk is still low in most situations. Although I suggest avoiding dog food and meat when it gets too low.
Dysentery
Caused by drinking non-potable water. Always boil or purify your water before drinking it.
Cabin Fever
Avoid spending too much time in shelters. Hinterland describes "too much" time as "the majority of six days". I assume that means it calculates the percentage of time you've spent indoors over the last six days and if it's above 50% then you get afflicted. Time inside includes all activities, not just sleeping. This is sometimes hard to avoid if you do a lot of crafting or other stuff indoors. If it's a problem for you, try building a snow shelter outside your door and sleep in it occasionally, instead of indoors.
Intestinal Parasites (Stalker and Interloper modes only)
Note: This is not a risk in Voyageur mode.
There's always a chance of this when eating game meat, cooked or not. The parasites build up over time and put you more at risk the more you eat. Don't eat too much meat in a day to avoid this.