The disease "Athlete`s Foot" is caused by a fungus.

Athlete`s Foot, also known as tinea pedis, is a common fungal infection that affects the skin of the feet, particularly the spaces between the toes. It is caused by various types of fungi, most commonly from the dermatophyte group. These fungi thrive in warm, moist environments such as sweaty feet and in public places like locker rooms and swimming pools.

The fungus responsible for Athlete`s Foot typically infects the outer layers of the skin, leading to symptoms such as itching, redness, cracking, and peeling of the skin. It can also cause blisters and a stinging or burning sensation.

To treat Athlete`s Foot, antifungal medications, such as creams, sprays, or powders, are commonly used to eliminate the fungal infection. Maintaining good foot hygiene, keeping the feet clean and dry, and wearing breathable footwear can help prevent the occurrence and spread of the infection.

Therefore, the correct answer is:

Fungus.