Rashes are skin eruptions that may be accompanied by pain, redness, swelling, blisters or itching. Many diseases, such as genital herpes and German measles, include a rash as a symptom of the underlying illness, while in other cases including contact dermatitis, the rash itself is the illness.
Viruses
Chickenpox and measles are examples of rashes caused by viral infections. Chickenpox, caused by the varicella-zoster virus, is a contagious disease that is easily passed through direct contact or from breathing the airborne virus after an infected person has sneezed. It presents as a widespread, extremely itchy rash with small, fluid-filled blisters. Once chickenpox has run its course, the virus remains in the body and can appear later in life as herpes zoster, or shingles. DermaNet NZ reports that measles is also highly contagious, and carries a high morbidity rate in people who have not been vaccinated. It presents with symptoms similar to those of the common cold. Approximately three to seven days after infection, a red, splotchy rash appears on the face and then spreads.
Bacteria
Certain bacteria cause skin rashes. Staphylococcus aureus can cause toxic shock syndrome, which is both severe and acute and which presents as a widespread red, flat skin rash that sheds, with complications in other organs possible. It was first noted in the late 1970's when a number of women developed the illness after using tampons. Since then, manufacturers have made changes to how tampons are produced and a majority of women have developed antibodies to the exotoxins released by the bacteria. Recurrence, however, is common in women who have been affected. DermaNet NZ estimates a 30 to 40 percent rate of recurrence. Streptococcus pyogenes is a similar bacterium that causes streptococcal toxic shock-like syndrome, or toxic strep. It produces a rash that may begin to shed about one to two weeks after infection, especially on the palms of the hands and the soles of the feet.
Irritants
A number of irritants are known to cause rashes in susceptible individuals. These include soaps and detergents, cleansers and cosmetics, and certain chemicals and waxes. These irritating substances break down the protective sebum layer on skin that is meant to keep out aggressors and to help skin to retain moisture. Irritant contact dermatitis, as the rash is known, presents as a rash with mild swelling, some cracking and possibly blisters.
Allergens
Over time, some people develop sensitivity to certain substances. Once that happens, the individual essentially becomes "allergic" to that substance and will ever after exhibit an allergic reaction when in contact with the allergen. Poison ivy, poison sumac and poison oak often cause allergic reactions, sometimes after the first exposure. Other allergens that cause skin rashes include gold, nickel, certain fragrances and sunscreens.
