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It's easy to understand how someone who gets hives for the first time could confuse the sudden appearance of raised, red, or skin-colored bumps with bug bites or other skin conditions, such as eczema. After all, the bumps appear suddenly, anywhere on the body, and can itch a little or a lot.

Unlike insect bites or eczema, however, hives come in different shapes and sizes -- from one or two pinhead dots to large swollen lumps that join together to form a shapeless rash or "wheal." Sometimes there is also swelling in the deeper layers of the skin, called "angioedema."

There are two main types of hives (which doctors refer to as "urticaria"), based on how long symptoms last. "Acute urticaria" refers to symptoms that come and go for up to 6 weeks, while "chronic urticaria" refers to symptoms that last longer than 6 weeks, often appearing daily or several times a week for a year or longer.

Acute Urticaria

Anyone can get urticaria, and it is seen in 20-25% of men, women, and children worldwide. The symptoms of acute urticaria, the most common type, most often affect young children, particularly boys, and usually disappear within 24 hours.

Acute urticaria can be caused by an allergic reaction to certain foods or medications, or to something that touches the skin, such as animal dander. It can also be caused in response to viral or bacterial infection, such as the common cold or the flu, strep throat, or even COVID-19. In about 50% of people who develop acute urticaria, however, the cause is never identified.

It's important to keep looking for the cause of urticaria to prevent it from returning or becoming chronic, said Alison Ehrlich, MD, of Foxhall Dermatology and Research Center in Washington, D.C. "Let's say you have an allergy to shrimp, but you don't know it, and you like to eat shrimp once a month. You start developing urticaria once a month, but it takes 6 months to figure out that you're allergic to shrimp. So you can kind of slide into chronic."

Keep working with your doctor to identify what's causing urticaria, Ehrlich advised. "The trigger may just show up later."

Chronic Urticaria

Chronic urticaria is much less common than the acute type, and is most often seen in adults between the ages of 40 and 60, with women affected twice as often as men. There are two distinct kinds of chronic urticaria: chronic spontaneous urticaria (CSU); and chronic inducible urticaria (CIndU).

Chronic Spontaneous Urticaria

CSU makes up 80-90% of all cases of chronic urticaria, but the cause is not known. It lasts for 1 to 5 years, sometimes longer, and does not appear to be related to external allergens or triggers. CSU is thought to be associated with autoimmune conditions such as thyroid disease.

Chronic Inducible Urticaria

Unlike CSU, there are many physical and environmental factors known to cause CIndU, and many different subtypes, with names that reflect the triggering factor:

• Dermatographism, the most common form of CIndU, literally means "to write on the skin" and is caused by scratching and rubbing that produces long red wheals

• Delayed pressure urticaria shows up 4-6 hours after wearing tight clothing, often at the waist

• Exercise-induced urticaria is triggered by physical activity

• Cold urticaria is caused by exposure to cold

• Heat urticaria is triggered by heat

• Solar urticaria is caused by exposure to ultraviolet light from the sun

• Vibratory urticaria is triggered by activities such as mowing the lawn

• Aquagenic urticaria can result from direct contact with water

• Cholinergic urticaria is caused by passively induced body heat and perspiration

• Contact urticaria is triggered by physical contact with certain substances, including fragrances and metals

Sources of Additional Information

Read Part 1 of this series: For Your Patients: All About Hives

"Medical Journeys" is a set of clinical resources reviewed by physicians, meant for the medical team as well as the patients they serve. Each episode of this journey through a disease state contains both a physician guide and a downloadable/printable patient resource. "Medical Journeys" chart a path each step of the way for physicians and patients and provide continual resources and support, as the caregiver team navigates the course of a disease.