Thorns are an environmental hazard in Alone: Zombiewoods. Aside from zombies, they are the only other "enemy" you can attack with weapons.
Thorns appear on the ground in the woods. They look like tall, green-and-yellow grass, and can be difficult to spot. Walking over thorns will cause your character to lose Health.
To avoid the Health loss, stay on the lookout for thorns. When you find some in your way, approach the thorns until you can strike them with your weapons. Then, attack until they disappear. Once the thorns have disappeared, you can move forward without fear of injury.