Plants also defend themselves with chemicals. Some chemicals are toxic to the pests that eat the plant, so animals learn to avoid it. Others are antimicrobial: they kill bacteria and fungi, or slow their growth, which helps protect the plant from pathogens.
Many of these chemicals affect human bodies too, so people have developed them into medicines. Willow bark contains a chemical that was developed into aspirin, which relieves pain and reduces fever. Foxglove makes chemicals that are poisonous to animals, yet a drug from the plant is used to treat heart conditions. Quinine from the bark of the cinchona tree is used to treat malaria.
A drug is given in a carefully measured dose, because the same chemical that protects the plant can be harmful in large amounts. Note that aspirin only relieves the symptoms of an illness. It does not kill the pathogen that causes it.