Ringworm is a fungal skin infection. There are many types of fungi and some can infect the skin, nails and hair. Scalp ringworm which is sometimes called tinea capitis.
Commonly, the infection just looks like severe dandruff on various places on the scalp. This may be mistaken for psoriasis.
Some infections cause patches of hair loss (bald patches) to develop.
Some people are carriers of the fungi. That is, they have fungi on their skin and hair but these do not invade into the skin to cause infection or symptoms. However, they may pass the fungi on to others who then do develop symptoms.