Diseases that cause an itchy rash on the chest include tinea versicolor, hives and prickly heat, according to the American Academy of Dermatology and Mayo Clinic. Tinea versicolor is caused by yeast, the AAD reports. The spots are dry, itchy and scaly and are either lighter or darker than the surrounding skin.
The colors of the spots of tinea versicolor range from white to brown, explains the AAD. Because of the yeast infection, they do not darken in color as the skin tans. The spots are slow growing, and after a time they join to form patches.
Hives are often the result of an allergic reaction to food or medication and can erupt anywhere on a person's body, the AAD reports. Sometimes they form welts that merge together over the skin. They typically fade after a day but are sometimes replaced by new welts. A case of hives can last for days but usually lasts less than a month and a half.
Though the rash of prickly heat is unusual on the chests of adults, it is fairly common on the chests of babies, according to Mayo Clinic. The symptoms are red, itchy bumps. Cooling the skin and keeping it cool usually resolves the condition.