Hives is also called urticaria, a harmless type of rash which is caused by an allergy. The rash or rashes typically appear circular, and raised on the skin. It is also itchy and occurs in batches. The size of hives vary from a few millimeters to several centimeters that can affect the throat, arms, legs and trunk.

Hives are caused by medications, food and insect bites. There are common triggers of hives that are identified. These are medications like antibiotics, codeine, penicillin, sulfa, anticonvulsant drugs, phenobarbital and aspirin. There are also certain foods that can trigger allergies like shellfish, nuts, tomatoes, soy, chocolate, and berries. Pollen from plant, cat dander, insect bites can also cause hives.

Emotional stress can also cause hives. Other conditions that can cause it are bacterial-viral-fungal infections, hepatitis B, parasitic worms, food additives, plants, cold temperatures, heat, sweating and dermatographism or irritation brought by wearing tight fitting clothes.

These are some ways to be able to ease the symptoms brought about by this conditions. Try to avoid heat as much as possible by keeping your skin cool. Avoid alcohol because this can cause flushing of the skin. Don't scratch the affected areas and wear loose clothing that will not put pressure on your skin.

Here's a quick line or two taken from WebMD. I've attached two links if you would like to read further.

"Hives and angioedema form when, in response to histamine, blood plasma leaks out of small blood vessels in the skin. Histamine is a chemical released from specialized cells along the skin's blood vessels."
"Allergic reactions, chemicals in certain foods, insect stings, sunlight exposure, or medicines can all cause histamine release. Sometimes it's impossible to find out exactly why hives have formed."

Hives are not contagious. I've attached 2 spots I read quickly. Rather informative.

Hives] often appears when a person's cell's have set off allergic reactions due to some medicines such as and other substances that may prompt the same. There have been three established culprits of Hives, these are foods, medicines and other factors such as bee pollen and bites from insects.

Accordingly, their are certain aspects in physical terms that can initiate hives, such as stress, exercise and "Dermatographia, this condition literally means "write on the skin." With regards to this condition, there is no indication that Hives is contagious, in the sense that it varies from person to person and it can be treated with medications with Antihistamine properties.

"If your symptoms are mild, you may not need treatment. The standard treatment for hives and angioedema is antihistamines, which block the symptom-producing release of histamine. For a severe attack of hives or angioedema, you may need an emergency injection of adrenaline (epinephrine) and a trip to the emergency room."