Scientists categorize ringworm as a dermatophyte, or fungi imperfecti, because its sexual life cycle is unclear. Most often, ringworm reproduces through membrane-like hyphae that form reproductive spores. These spores invade the outer, dead portions of skin and other areas where there is keratin, such as in hair and nails.
In humans, ringworm infections are caused by the fungus trichophyton and typically occur on the feet, especially between the toes. The rate of spore growth depends on the presence of nutrients in the environment, but germination typically occurs within 6 hours of exposure, and small colonies form within 24 hours. By 48 hours, the spores enter the stationary phase, during which growth slows, and they begin to produce enzymes that allow them to invade the epidermis. Because this layer of skin has very little blood supply, the body’s immune system is unable to fight the infection effectively at this stage. The spores remain in the skin indefinitely, which is one reason why ringworm infections tend to be resistant to treatment and often recur.
Up to one-third of ringworm infections cause no symptoms. However, when the enzymes begin to degrade the epidermis, sometimes an inflammatory response results. It is this inflammation, not the ringworm itself, which causes the burning, itching and rash often associated with ringworm.