A. DRUG REACTION: Drugs (substances added to the body to cure diseases or help with symptoms) can have unexpected and unintended effects on the body. People can be born allergic to particular drugs, or can become allergic. Or an underlying syndrome can make the body's immune system respond with a host of symptoms -- the airways can close, blood supply can be affected, the skin can react -- causing shock, fever, even death.
B. POISON IVY: Contact with an oil (urushiol) found in poison ivy, oak or sumac can cause a skin rash, blisters, and hives in those that are sensitive to it. The oil is in all parts of the plants: leaves, stems, flowers and roots. You don't even have to touch the plant directly -- if you touch clothing, pets or anything else that is contaminated by urushiol, you may have the reaction. But urushiol does not cause a rash on everyone who gets it onto his or her skin.
C. VIRUS: A virus is a nonliving agent that can reproduce itself and infect the body. Common viruses can cause anything from colds or flu to chickenpox, pneumonia, hepatitis and ear infections. Symptoms of viruses are as varied as the viruses themselves -- skin blisters, lung and sinus congestion, pain, and in more severe cases, brain inflammation. Antibiotics do nothing against viruses; but there are medicines to treat some viral infections.
D. SKIN PARASITE: A skin parasite uses human skin as a host. A fungus, for example, could live on human skin (this is the cause of "ringworm") and cause irritation, or an insect could use the skin as a breeding ground. For instance, scabies is a skin ailment caused by a mite, and leads to intense itching, redness, and swelling. Lice cause similar conditions.