Toxicology is the science of poisons, which are sometimes referred to as toxins or toxicants. The former term applies to all natural poisons produced by organisms, such as the botulinum toxin produced by the bacteria Clostridium botulinum. The latter more generic term includes both natural and anthropogenic (human-made) toxicants like dichlorodiphenyl trichloroethane (DDT), which is perhaps the most commonly recognized toxicant.

Even though the botulinum toxin is extremely toxic to humans, and DDT is relatively toxic to insects, it is important to recognize that virtually any element or compound will become toxic at some concentration. For example, iron, which is an essential component of hemoglobin, can cause vomiting, liver damage, and even death if it is ingested in excess. This concept of toxicity was recognized five centuries ago by the Swiss alchemist and physician Paracelsus (1493–1541), who stated that, "The right dose differentiates a poison from a remedy." How...