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An exotoxin is a poison secreted by an organism like a fungus, bacterium, alga, or protozoan. Exotoxins are infamously virulent. A very small amount can be fatal to a host organism and even though the immune system can often identify and attack the toxin, the toxin spreads so quickly that the host does not have an opportunity to mount a defense. Some governments historically have attempted to harness microorganisms that produce toxins in warfare, and the development of weaponized microorganisms led to the creation of a treaty banning biological warfare out of concern that such organisms could get out of control.
Some organisms secrete exotoxins steadily, while others produce them as needed, and in some cases they are only released during lysis, when a cell breaks up as an organism dies. They are usually proteins that interact with proteins and enzymes found in the body of the host. An exotoxin can be classified by the types of tissues it targets, like neurotoxins that target neurons and enterotoxins that are designed to assault the digestive tract.
Using an exotoxin, a microorganism can attack remote areas, rather than needing to be in direct contact with the targeted tissue. The exotoxin can enter the bloodstream and travel, using the body's own circulatory system as a delivery method. Some are designed to assist with bacterial invasion, as with exotoxins that break down tissues to allow organisms to penetrate more deeply, while others do not have a known function.
Individuals with infectious diseases associated with exotoxins are at serious risk. These toxins can lead to widespread tissue death, known as necrosis, in some cases necessitating amputation to stop the spread of the tissue necrosis and save the patient's life. If exotoxins target vulnerable organs like the brain, permanent damage can be done. Even if the patient's infection can be treated, lingering complications may occur as a result of the damage caused by the exotoxin.
Medications are available to treat people with severe fungal, bacterial, viral, and protozoan infections. These medications are designed to kill the microorganisms or prevent them from reproducing. If available, antitoxins can be administered to offset the effects of exotoxins and increase the patient's chance of survival. These compounds are produced naturally by a number of organisms can be given to a patient with a known infection to counteract the toxins associated with that infection. Not all exotoxins have a corresponding antitoxin, however.