Red Dead Redemption 2 features snakes. Watch out when you encounter one - they can attack Arthur. If they bit you, their venom gets into your bloodstream. This venom will slowly deplete your health. On this page you can learn how to deal with being beaten by a snake.
Drink a medicine
Drink Potent Health Cure to get rid of venom from your body. A cure can be bought at a store or it can be crafted by Arthur. You need 1 Ginseng. The recipe is available from the beginning of the game.
After drinking the cure, you should see the text in the upper left corner (the same as the one above). If not, then you should go to sleep.
Keep healing until the venom wears off
If you don't have a Potent Health Cure and you don't carry any ingredients, then you should keep healing until the poison wears off. Replenish your entire health bar and go to sleep. After a few days, the poison should wear off.