Clayton Cowan, 7, and his younger brother Graham, 5, had big hugs for the Six Flags America characters who greeted them when they arrived in the Baltimore/Washington area theme park earlier this month. Bugs Bunny and Sylvester returned the favor.

The brothers and their mother, Marty Cowan, had stopped by to meet officially some of the park's inhabitants and employees who would welcome them and fellow walkers in its upcoming "Walk in the Park, Taking Strides to Cure Kids Cancer"  fundraiser, June 13, 2010, to benefit Hopkins Children's. Clayton is this year’s “walk ambassador,” representing all the children in the hospital’s care.

A resident of Fulton, Md., Clayton was born with chronic granulomatous disease (CGD), a disease so rare it occurs in only one out of a million people. Caused by a glitch in the immune system, CGD makes children like Clayton very vulnerable to germs, leading to recurrent chronic infections, some of which can be life-threatening.

He was not diagnosed until, at the age of 2, he had a severe lung infection and was taken to Hopkins Children’s. There, “the brilliant Dr. Howard Lederman had a diagnosis for us within 48 hours,” says his mother. Hopkins hospitalized the toddler for three weeks, while doctors figured out the appropriate course of treatment.

Read Clayton's story and course of treatment at Hopkins Children’s.

Listen to him and his mother talked about his illness during a 2008 Radiothon  interview. 

Register to walk or sponsor a walker at Six Flags America, June 13. Anyone who raises $50 receives a free ticket to the park that day.