Fungal infections can affect the nails of fingers and toes and are contagious, according to WebMD. An untreated infection may spread to other fingers or toes, other parts of the skin or other people.
Fungal infections of the nail, called onychomycosis or tinea unguium, are common and do not usually occur in all or multiple fingers at once, explains Mayo Clinic. There are many ways the fungus is transmitted to other people, including direct contact with shared surfaces. An individual with fingernail fungus may also spread it to others through improperly cleaned grooming equipment such as nail clippers, notes Healthline.