Hormesis is a dose–response relationship characterized by a biphasic dose response to stressors with a low dose stimulation and a high dose inhibition. The first systematic description of hormesis appeared toward the close of the 18th century by the German pharmacology professor Hugo Schulz. The stressor agent can be any agent or factor capable of causing a deleterious effect. The biological systems can be diverse: bacteria, fungi, algae, yeasts, animals, humans, protozoa and plants. The range of endpoints covers longevity, reproduction, cancer, survival, growth, metabolic effects and others.