Infection on the skin of the feet may cause athletes foot and in the groin, jock itch. Involvement of the nails is termed onychomycosis, and they may thicken, discolour, and finally crumble and fall off. They are common in most adult people, with up to 20% of the population having one of these infections at any given moment.Animals including dogs and cats can also be affected by ringworm, and the disease can be transmitted between animals and humans, making it a zoonotic disease. Specific signs can be:

red, scaly, itchy or raised patches
patches may be redder on outside edges or resemble a ring
patches that begin to ooze or develop a blister
bald patches may develop when the scalp is affected
nails may thicken, discolour or begin to crack

Causes
Fungi thrive in moist, warm areas, such as locker rooms, tanning beds, swimming pools, and skin folds; accordingly, those that cause dermatophytosis may be spread by using exercise machines that have not been disinfected after use, or by sharing towels, clothing, footwear, or hairbrushes. Diagnosis
Dermatophyte infections can be readily diagnosed based on the history, physical examination, and potassium hydroxide (KOH) microscopy. Classification
A number of different species of fungus are involved in dermatophytosis. Dermatophytes of the genera Trichophyton and Microsporum are the most common causative agents. These fungi attack various parts of the body and lead to the conditions listed below. The Latin names are for the conditions (disease patterns), not the agents that cause them.
Individuals ages 21–40 years are most commonly affected.A 2002 study looking at 445 samples of dermatophytes in patients in Goiânia, Brazil found the most prevalent type to be Trichophyton rubrum (49.4%), followed by Trichophyton mentagrophytes (30.8%), and Microsporum canis (12.6%).A 2013 study looking at 5,175 samples of Tinea in patients in Tehran, Iran found the most prevalent type to be Tinea pedis (43.4%), followed by Tinea unguium. (21.3%), and Tinea cruris (20.7%). See also
Mycobiota

References
Further reading
Weitzman I, Summerbell RC (1995). "The dermatophytes". Clinical Microbiology Reviews. 8 (2): 240–259. doi:10.1128/cmr.8.2.240. PMC 172857. PMID 7621400. External links

Tinea photo library at Dermnet