Sarah Chalke's Very Personal Grey's Anatomy: Episode Inspired by Son's Deadly Disease

Exclusive! Find out what happened when actress's little one battled a rare and sometimes fatal illness

Sarah Chalke has a very personal reason for being on Grey's Anatomy tonight.

Chalke decided to pitch herself to producers after her 3-year-old son Charlie was diagnosed a year ago with Kawasaki disease, a rare and sometimes fatal autoimmune illness.

"It's a very visual disease," Chalke explained. "It's a high fever, and then everything because of inflammation goes red. So it's red, bloodshot eyes, red lips, red rash and red hands and feet, and then the last telltale sign is the skin on the fingers peels back. It was absolutely the hardest thing I've ever been through, and there's nothing in the world like not knowing if your kid is going to be OK."

While scientists are unsure of what causes Kawasaki disease, about 4,200 children are diagnosed with the illness every year in the U.S.

"There is a treatment, but basically you only have a 10-day window to get it," Chalke said. "If you don't get that, there's so much inflammation in the body that the danger is that you can get aneurysms because there can be damage to the coronary arteries, and then the kid can have a heart attack."

After Chalke went public with Charlie's story at a fundraiser for the Kawasaki Disease Foundation in November, she decided she wanted to do something on a larger scale to raise awareness. "Someone suggested doing an episode on a medical show on television where you can reach millions of people in one night," Chalke said.

She immediately contacted Grey's creator Shonda Rhimes, who jumped on the idea. Fast-forward to tonight's episode—Chalke plays a single mom of a 4-year-old boy fighting for his life when he develops Kawasaki disease.

Chalke admits she was nervous going into the shoot, especially about seeing a little boy made to look like he had the horrible illness.

But then she got a sign that everything was happening for a reason. One of the three triplets cast to play her son had Kawasaki disease a year ago. "Casting didn't know this, and his mother didn't know the storyline until after she accepted the part and read the script the night before the shoot," Chalke said. "What are the chances of that happening? Zero! It made me feel like, OK, this was all meant to be."

To learn more about Kawasaki disease, go to KDFoundation.org.