Actress Mandy Moore revealed on Instagram last week that her 2-year-old son, Gus, had developed a case of an uncommon rash called Gianotti-Crosti syndrome.

“This sweet boy woke up with a crazy rash on Saturday (morning),” the “This Is Us” star wrote in an Instagram Story. “We tried to deduce what it could be and did anything to help him find relief from the itch.”

Moore took her son to urgent care, a pediatrician and two dermatologists before Gus received the rare diagnosis for the itchy bumps go Gus’ arms, legs and feet.

The National Institutes of Health describes Gianotti-Crosti syndrome as a skin condition in children that usually surfaces in the aftermath of an underlying infection, most often a virus. In the U.S., it may occur after a child has Epstein-Barr virus, a common illness that’s spread through bodily fluids and includes symptoms like fever, fatigue, sore throat and swollen lymph nodes. The virus is the most common condition associated with infectious mononucleosis, or “mono.”

Apart from Epstein-Barr, Gianotti-Crosti syndrome is sometimes caused by other viruses including enterovirus, adenovirus and rotavirus. In other parts of the world, it’s more often associated with hepatitis B. This is less of a concern in the U.S. due to higher rates of vaccination. Rarely, the rash will develop in response to a vaccination for the flu, polio and other diseases.

Blisters from Gianotti-Crosti syndrome may appear on the legs, arms, buttocks and face. They’re usually large and fluid-filled with flat tops. The lesions may or may not itch, and they can last anywhere from 10 days to a few weeks or months before they resolve on their own.

When the rash doesn’t go away, a pediatric dermatologist may choose to prescribe a topical steroid to help eliminate it. Parents can also use over-the-counter hydrocortisone and antihistamines to help relieve symptoms.

Although Gianotti-Crosti syndrome sometimes occurs in adults, it’s usually found among children between 1 and 6 years old.

The challenge for parents and doctors is properly identifying the rash, since it’s not as common as other skin conditions and illnesses that show up in kids.

“I would say that in general this rash might be a little bit more tricky for non-dermatologists to diagnose,” Melissa Levoska, an assistant professor of dermatology at the Icahn School of Medicine at Mount Sinai in New York City, told CNN. “I don’t think all physicians have seen it frequently.”

Levoska said it’s a good idea for parents to take their young children to see a dermatologist when they have rashes, since the causes may be overlooked and some conditions may be mistaken for others.

Kids with Gianotti-Crosti syndrome are not at risk of long-term complications, but Levoska said the skin in the area of the blisters may be discolored for a period of time before it heals.

“This parenting thing is weird and hard and sometimes you feel so helpless,” Moore said. “As long as he is smiling through it, we are ‘A’-okay.”