Ringworm is not a worm, it is a fungus, it loves a dense coat in humid weather, and it spreads to people -- particularly children and anyone whose immune system is already busy.
It shows as circular patches of hair loss with crusty, rough edges, often starting on the face, ears or paws. Under a UV lamp some strains glow, though not all, so a negative lamp test proves nothing.
Treatment is usually a combination: an antifungal applied to the skin, sometimes an oral medicine, medicated baths, and -- the part everyone skips -- decontaminating the house. The spores survive in bedding, carpets, brushes and the sofa for a long time.
If you have been through it:
- How long did it actually take to clear?
- What did your vet prescribe?
- Did anyone in the house catch it?
- How did you handle the cleaning, and did you confine the cat to one room?