Pyrimethamine is a medicine that helps to fight against a type of parasite called Plasmodium that can make people sick with a disease called malaria. Imagine that your body is like a kingdom, and there are some harmful parasites that want to attack and harm your kingdom. Pyrimethamine is like a shield that helps to protect your kingdom by killing these harmful parasites. This medicine stops the parasites from growing, and it makes them weaker so your body can fight them better. It is usually taken with some other medicines to kill the parasites and make you feel better. Pyrimethamine can be very helpful, but you must follow the doctor's orders and take it exactly as they tell you to.