Pesticides are substances that prevent, destroy, repel, or reduce the severity of pests. Pests are living things that occur where they are not wanted or that cause damage to humans, crops, or animals. Pests can be insects, rodents, unwanted plants, bacteria, viruses, or different types of fungus. Pesticides can vary in how toxic they are to humans and the environment. Some are persistent in the environment, animals, and birds, lasting for years; others break down soon after they are released. The U.S. Environmental Protection Agency (EPA) grants licenses, or registrations, to pesticides that it has found do not pose unreasonable risks to human health and the environment; it has registered at least 865 pesticides, which are used in thousands of pesticide products.