Ringworm, known medically as tinea, is a common and contagious fungal infection of the skin, hair, or nails. A dermatophyte fungus causes the infection by feeding on the keratin in the outer layers of the skin, despite the misleading name. The timeline from initial contact to visible symptoms and the duration of contagiousness are highly variable, depending on the source of the infection and the host’s body.
The Incubation Period: From Exposure to Symptoms
The incubation period, the time from exposure to noticeable symptoms, typically ranges from four to fourteen days. For ringworm affecting the body (tinea corporis), the first visible sign often appears within four to ten days. Scalp ringworm (tinea capitis) tends to have a slightly longer incubation period, usually becoming apparent about ten to fourteen days after exposure.
A person’s immune system health plays a role, as a weakened immune response can shorten the incubation time. Skin integrity is also a factor, as cuts, scrapes, or abrasions provide an easier entry point for the dermatophyte spores. The source of the infection—whether from an infected animal, another person, or contaminated soil—can also slightly alter this initial timeline.
Progression Rate of the Ringworm Lesion
Once the fungal spores breach the skin barrier, the resulting lesion begins a characteristic pattern of centrifugal expansion. The rash starts as a small, discolored, and often scaly patch before gradually enlarging outward over the skin. The fungi colonize the skin’s outer layer, causing inflammation that is most active at the lesion’s edge, which appears raised, red, and scaly.
The expansion is slow and steady, with the fungi spreading peripherally while the center of the rash often begins to clear, leading to the classic ring-like shape. Lesions commonly range in size from one to five centimeters, and untreated spots can continue to grow over weeks or months. The rate of spread across the body can be faster in moist, warm areas, such as the groin or feet, which offer a more favorable environment for fungal growth. Scratching the infected area can also lead to autoinoculation, where fungal spores are transferred to adjacent skin or other body parts, causing new lesions to appear elsewhere.
How Long Does Contagiousness Last?
Ringworm remains highly contagious for as long as active fungal spores are present on the skin, which can be for several weeks or months if left completely untreated. The infection spreads through direct skin-to-skin contact with an infected person or animal, or indirectly by touching contaminated surfaces. For a person receiving treatment, the risk of transmission drops significantly within a short window.
A person is generally considered non-contagious to others about 48 hours after starting an appropriate topical or oral antifungal medication. However, the patient must continue the full course of treatment, often for two to four weeks, even after the rash appears to have cleared completely. Stopping treatment prematurely risks leaving viable spores behind, which can cause the infection to recur and become contagious again.
The environment itself can also harbor viable spores for a surprisingly long time, acting as a source of reinfection or transmission. Fungal spores can survive on objects like clothing, towels, bedding, combs, and shower floors for weeks or even months. Some resilient species of dermatophytes have been documented to survive in the environment for up to 18 to 20 months. Cleaning and disinfection of contaminated items are necessary to break the cycle of transmission.