Dry, itchy, scaly red skin - it's the hallmark of eczema, sometimes called atopic dermatitis.
It's most common in children and infants, but adults can have it, too.
Doctors can treat it, but they never really understood how it happens - until now.
Milla George is a little girl who's had a big eczema problem ever since she was born.
"Within about a week of her being born, it was red, scaling, itchy - face, body, hands - pretty much everywhere, " recalls Milla's mother Christina Chung.
Dermatologist Dr. Herbert Allen of Drexel College of Medicine says about 1 in 5 people has eczema, and the number is rising.
There's no age limit to the troublesome condition.
"Kids tend to outgrow it, but it can come back later on," Dr. Allen says.
He says doctors have known for years that genetics plays a role - some, like Milla, are more prone to it.
And doctors knew that staph, a common skin bacteria, also plays a role.
Now, his team has discovered how it happens.
The staph mixes with salt in sweat and blocks the sweat ducts.
"The sweat couldn't get out, so that triggers a reaction by the immune system, and then that in turns makes all the inflammation and the itching," says the doctor.
He says this helps explain why the itch of eczema starts before the actual rash.
While you can't change your genetics, people who are prone to eczema can take steps to prevent it.
"We stress being really gentle with the skin care. Minimal amount of soap, mild soap at that," he says.
Mild moisturizing bar soap is the best, according to Dr. Allen. He says liquid and gel soaps dry out skin.
And although long, hot showers are relaxing, they strip the skin of its natural defenses, so try NOT to use hot water.
Also, slather your skin with moisturizer, as often as you can. Dr. Allen doesn't suggest any particular type.
"Whatever feels good," he says.
But make sure it's fragrance free.
Milla's mom also using humidifiers in a child's bedroom during sleep and naptime help.
She says one helped stop an eczema flare-up for her 5-month-old son.
"Within a day or two, he was much better, and nothing else had changed," she says.
Dr. Allen is now looking into skin diseases which may operate with the same mechanism or which have eczema in their symptoms, such as seborrheic dermatitis, prickly heat, and acute athlete's foot.