Canker refers to many different diseases of such broadly similar symptoms as the appearance of small areas of dead tissue, which grow slowly, often over years. Some are of only minor consequence, but others are ultimately lethal and therefore of major economic importance. Their causes include such a wide range of organisms as fungi, bacteria, mycoplasmas and viruses. The majority of canker-causing organisms are bound to a unique host species or genus. Weather and animals can spread canker, thereby endangering areas that have only slight amount of canker.