Poison ivy, poison oak and poison sumac are plants that contain an irritating, oily sap called urushiol. Urushiol triggers an allergic reaction when it comes into contact with skin, resulting in an itchy rash, which can appear within hours of exposure or up to several days later.
Polymorphous light eruption. Patients with this condition develop papules, papulovesicles, or erythematous plaques in response to sun exposure. The lesions erupt a few hours to several days after the subject has been exposed to sunlight. Lesions are most often located on the face, upper chest, and exposed parts of the extremities. Ocular inflammation and cheilitis may also occur. Among North American and Latin American Indians, polymorphous light eruption tends to be a familial disease with childhood onset.
Rocky Mountain spotted fever. This disease is caused by Rickettsia rickettsii and is transmitted by a number of different ticks. Despite its geographical title, Rocky Mountain spotted fever is present in many locations throughout the United States and the entire Western Hemisphere. After infection by tick bite, there is an incubation period of 2–14 days. The abrupt onset of the disease includes severe headache, fever, chills, arthralgia, and myalgia. After 2–3 days of these constitutional symptoms, erythematous macules erupt on the wrists, hands, forearms,legs, and ankles, as seen in these figures. Lesions then spread to the palms and soles and the trunk. The macules originally blanch with pressure but soon become purpuric and even necrotic. The disease causes a severe vasculitis and complications include disseminated intravascular coagulation, hemorrhage into the gastrointestinal and urinary tracts, and cardiovascular collapse.