Ringworm is not a bacteria. Despite its name, ringworm is a fungal infection caused by a group of fungi called dermatophytes. There is no worm involved either. The name comes from the ring-shaped rash the infection typically produces on the skin.
What Actually Causes Ringworm
Ringworm is caused by three main groups of fungi: Trichophyton, Epidermophyton, and Microsporum. These are specialized fungi that have evolved to feed on keratin, the tough protein that makes up your skin, hair, and nails. One species in particular, Trichophyton rubrum, accounts for 80% to 90% of all ringworm infections and has been the dominant strain for over 70 years.
These fungi break down keratin in a two-step process. First, they dissolve the strong chemical bonds holding keratin together. Then they use an arsenal of protein-digesting enzymes to chop it into smaller pieces they can absorb as food. This is why ringworm stays in the outermost layers of your body, the parts made of keratin, rather than spreading deeper into tissue the way a bacterial infection might.
Why the Distinction Matters for Treatment
If you treat ringworm with antibiotics, nothing will happen. Antibiotics target bacteria and have no effect on fungi. Ringworm requires antifungal medication, and using the wrong treatment gives the infection more time to spread and potentially pass to others.
Most ringworm on the body, feet (athlete’s foot), or groin (jock itch) clears up with over-the-counter antifungal creams or ointments applied for two to four weeks. Common active ingredients in these products include clotrimazole, miconazole, terbinafine, and ketoconazole. Ringworm on the scalp is harder to reach with a cream, so it typically requires prescription antifungal medication taken by mouth for one to three months.
How Ringworm Looks Different From Bacterial Skin Infections
The confusion between fungal and bacterial infections is understandable because both cause red, irritated skin. But their appearance differs in ways you can usually spot. Ringworm produces a distinctive ring-shaped rash: the border is raised, red, and scaly, while the center often looks clearer or relatively normal. It tends to expand outward over time.
Bacterial skin infections like impetigo look quite different. Impetigo starts as small red spots that develop into blisters, which rupture and ooze fluid before forming a thick, honey-colored crust. Bacterial infections like MRSA often appear as a swollen, warm bump that may be filled with pus. Swollen lymph nodes near the infection site are another clue pointing toward bacteria rather than fungus. Ringworm rarely causes swelling or pus unless it has become secondarily infected with bacteria from scratching.
How Ringworm Spreads
Ringworm is highly contagious and spreads through direct skin-to-skin contact with an infected person or animal. Pets, particularly cats and dogs, are common carriers. You can also pick it up from contaminated surfaces. Fungal spores shed onto clothing, bedding, towels, and shower floors can survive for 12 to 20 months as long as they have dead skin cells to feed on and a warm, moist environment.
To lower your risk:
- Wear protective footwear in public showers, locker rooms, and pool areas
- Never share towels, clothing, or bedding
- Dry thoroughly after showering, especially between your toes and in the groin area
- Wear breathable cotton underwear to reduce moisture buildup
When Bacteria Do Get Involved
While ringworm itself is purely fungal, scratching the itchy rash can break the skin and open the door to a secondary bacterial infection on top of the fungal one. Signs that bacteria have entered the picture include increasing pain, warmth, swelling, pus, or spreading redness beyond the original ring. In that situation, you may need both antifungal and antibacterial treatment, but the underlying ringworm still requires an antifungal to resolve.